Abuse of Power is the reason.
It's an "other high crime or misdemeanor".
Clearly, it's not a crime, so I take it your answer is that it is a misdemeanor - the "catch all" phrase that allows for everything the House wants.
I do not necessarily disagree - and I certainly agree it is not a necessity for a crime to have been committed (thus the catch all wording). What I was looking for is clarification. MidnightRider said the list was not a complete list; literally he argued it's a constitutional process, then argued the constitution was not complete in what is on the list of things. Obviously, that's idiotic. Your argument that it is a misdemeanor at least has a constitutional basis. It's arguable, but I would be on your side of that argument.
Let's look at a hypothetical example no one else is willing to discuss:
Trump is found guilty of a federal crime based on the Mueller report. Trump, as President, pardons himself. There's no law broken there, but is that not an abuse of power worthy of impeachment?
If Trump is alleged to have committed a crime, the constitutional answer to that, by congress, is impeachment. If he's found guilty and removed from office, then he could be tried in court over it (specifically what the constitution allows for, and suggests is what should happen).
Because that is the process, Trump could not pardon himself. He would no longer be president. He specifically cannot pardon himself from impeachment (again, spelled out clearly in the constitution).
So, your hypothetical has procedural misunderstandings.
Let's take it to something more reasonable - let's pretend there's actually evidence that Trump said, "I'm going to use the power of my office to harm my political opponent. I'm not going to tap his phones, I'm going to use the intelligence community to set up a guy he might talk to on the phone, tap that guy's phones, then when he talks to Trump I'll have reason to tap Trump's phone, but only for the purposes of watching the other guy (wink, wink)". THAT would actually be an abuse of power. THAT would be, arguably, the misdemeanor category and impeachable. Not necessarily a crime that one could put a pin into (maybe FISA court abuse, but that would be underlings, for example). But, abuse of power that would justify impeachment.
Would you believe that allowing another country to seize US territory was not an impeachable offense either? Because there's no statutory law that doesn't "allow" Russia to re-take Alaska? That's Dershowitz's belief and I think that's wrong. I believe that limiting impeachment solely to statutory crimes and misdemeanors allows Presidents to arguably violate the Constitution without issue.
Trying to re-take Alaska would be a geopolitical thing that would involve a lot more than a president. That's what Congressional authority for war is for. The president does not have the authority to just sign it over. That's why Obama's deal with Iran was meaningless - it was never a ratified treaty.
But, I would say that violating the Constitution is, in and of itself, a crime. When FDR rounded up people because of the slant of their eye and the coloration of their skin - imprisoning US citizens for racist reasons - was worse than impeachable. But, that would be a high crime, not a misdemeanor or "arguably" a crime - it IS a crime.