First and foremost because Trump has a long, long history of lying under oath.

What is most disturbing in Trump’s sworn statements is the amount of nonsense he spouts as he mangles the English language into meanings no rational person could accept. An unsuccessful “development by Donald Trump” is not a “development by Donald Trump.” A successful project built by another developer who paid to have Trump’s name on the building is a “Donald Trump development.” A payment of $400,000 equals a payment of $1 million. An ownership stake of 30 percent is actually a 50 percent stake. In a single sentence, he says he knows some people’s names but not their identities, as if talking about Batman and Superman. He studied résumés, but he only glanced at them. The list goes on, with one point in common: Every one of his answers, while under oath, depends not on the truth but on whether it makes him look good.

And that ladies and gents is exactly the problem.  Trump is incapable, even under oath, of saying something that makes him look bad.  Even if only a little bit bad.  He weasels his way around words, facts and meaning like a snake.  He’s done this for years, largely in civil settings and in many of those cases those lies have cost him dearly and expensively — like his $25 Million settlement in the Trump University case where he was represented by the same attorney his is now — but this time, it could very well cost more than a few $Million, it could cost him the Presidency and ultimately, his freedom.

He lost those cases then, he’s likely to lose this one now.

Here's another example.

Trump often doesn’t even try to make sense when explaining away a lie. In 2011, he was deposed about a failed Florida condo project. The building’s developer had paid a licensing fee to slap the Trump name on it, but—other than allowing his name to be used in marketing to deceive potential buyers—Trump had nothing to do with the project, which closed after taking hundreds of thousands of dollars in nonrefundable deposits. During Trump’s testimony under oath, the plaintiff’s lawyer confronted him with marketing material in which he had boasted that the building would be a “signature development by Donald J. Trump.” Despite the indisputable meaning of those words, Trump disputed them: When the advertising says the building is a development by Donald Trump, “in some cases they’re developed by me and in some cases they’re not.” He never explained how “developed by Trump” can mean “not developed by Trump” but pointed out that the lengthy legal documents signed by those unfortunate buyers disclosed in the fine print that he was not the builder. Why, then, the plaintiff’s lawyer asked, didn’t he include that disclosure in the advertising rather than the misleading “signature development” clause? “You can’t put it in the advertising because there’s not enough room,” Trump replied.

And this one…

The all-time classic Trump deposition is the one he gave in 2007 in a libel lawsuit he brought against Timothy O’Brien, author of TrumpNation, because the book stated that Trump’s net worth was far less than he claimed. (It was. Just ask Deutsche Bank.) Throughout this deposition, Trump sounded delusional, in what some might dismiss as compulsive lying. But knowing Trump, I don’t think he was lying; he believed what he was saying, but the facts just kept getting in his way.

Trump needed to prove he was damaged by the purported libel, but he wasn’t content with just saying he had lost some specific bit of business. Instead, he claimed to have lost business he never knew existed. “The fact is that a lot of people who would have done deals with me didn’t come to do deals with me,” he testified. “I can’t tell you who they are because they never came to me.”

Then there were the questions about what he owns. Trump was shown a nasty note he had written to a reporter in which he claimed to own 50 percent of a Manhattan property called the West Side Yards. In fact, he owned 30 percent, but rather than simply say he’d made a mistake, Trump claimed 30 percent equals 50 percent. “I own 30 percent,” he testified. “And I’ve always felt I owned 50 percent.” The reason, Trump explained, was that he didn’t put up any of his money in the deal, an explanation that makes no sense and does not change the fact that 30 percent is not, nor never will be, 50 percent.

We have to remember to obstruction of justice was used against Nixon as an article impeachment because he said “Uh huh" when one of his staff suggested someone tell the CIA to lie to the FBI to get them to stop investigating Watergate.  Trump is alleged to have cut all the middle men out of the picture and done this himself.

Articles of impeachment where issued against Bill Clinton for saying “I did not have sexual relations with that woman” when what he was referring to was a specific list of “sexual relations” that the Paula Jones attorneys had given him.  His testimony in that case, and in to the Grand Jury was that he to the best of his recollection didn’t do any of the specific things -- "touching in order to gratify” — on the list.  The Special Prosecutor then pretty much published a soft-core porn novel just to prove that he was “lying” because he once, according to Monica, “brush the back of his hand against he breast” — which he frankly might not have remembered —  and also the thing with the cigar. Although even with that, he wasn't doing the touching, the cigar was touching.  Frankly i think he was that circumspect and not laying his hands on her on purpose since he was a lawyer and already knew about those types of “sexual relations” definitions. IMO He was wise enough to keep himself within a stones throw of “plausible deniability” at all times.

Technically speaking, IMO because he was so careful, Clinton wasn't lying and he didn't perjure himself.  He later only admitted to making some minor mistakes, and even at that nothing involving Monica was actually material to what Jones was alleging.  Bringing up Monica was an attempt to show a pattern of behavior on Clinton’s part even though the patterns were pretty different between Jones and Lewinski — it was Monica who came on to Clinton in the first place by flashing her thong, not the other way around. 

Either way the “He said/She said” of most of this was resolved by the fact that Monica’s complaints about their relationship were [illegally] recorded by her friend Juillane Golberg.  Those recordings plus the blue dress DNA were what was used to get Clinton impeached, although not removed or prosecuted.

But Trump isn’t as smart, careful or as legally savvy as Bill Clinton.  If things get to a similar stage I doubt Trump’s presidency will survive as Clinton’s did.

In the current scenario Trump is claiming that James Comey lied in front of Congress when he claimed that Trump said “I need loyalty” and also when he said “Could you let Flynn go?”  He’s also made claims that Comey is a “leaker” but that’s all nonsense and dribble-drabble so let’s leave that aside.

I suppose Trump and his attorney imagine that the opportunity for this testimony will come as a result of their complaint to the DOJ inspector general regarding Comey and Senate Intelligence committee, but the much more likely that it will be with Robert Mueller’s investigators but he faces several problems with that.

Mueller has already spoken to Comey, we know that because Comey had been scheduled to testify earlier, but had it pushed back so that he could speak with Mueller first, and one of the first things Mueller did when arrived at his DOj offices was to review Comey's memos about Trump

Because of Comey’s Senate testimony we know a lot of what is likely contained in these memos, but we don’t know everything.  Any truly good investigator or prosecutor is not going to give up his best evidence until he can use it most effectively, so I would strongly suspect the reason Meuller hasn't honored congresses request for copies of the memos is he’s waiting to question Trump on them.

However, before he does that he’s going to question others who were potential witnesses to these events, particularly Comey’s own staff who were apparently in the room for Trump’s last phone call where he hinted back at exactly what he now claims never happened.

On March 11th Trump called Comey at his FBI offices:

He said he would do that and added, “Because I have been very loyal to you, very loyal; we had that thing you know.” I did not reply or ask him what he meant by “that thing.” I said only that the way to handle it was to have the White House Counsel call the Acting Deputy Attorney General. He said that was what he would do and the call ended.

In this call Trump alludes to “that thing” and he specifically mentions “loyalty” in that context so he’s effectively confirming what Comey has said about their two one-on-one face to face conversations. This call happened according to his testimony to the Senate while other members of his staff were in the room, and he immediately began to discuss the call with them.

So what we have isn't just “He said” vs "He said" - we have Trump and his various history of flim flammery about facts and even his own prior statements in public vs Comey and his personal integrity, his contemporaneous notes, his conversations about Trump’s statements with his staff and with his friends.