Because as LGM points out, whatever deals Trump cut with Russia, the Russians know about them. Which positions them to pull all kinds of blackmail games. Plus whatever deals may have been struck (case in point).
And what we learn about it will be worse than we already know. Because it’s a given the Trump administration is hiding stuff still. That’s why they’re trying to block the investigation, and talking about whether Trump can just pardon everyone involved.
I’m inclined to agree with a number of liberal bloggers (only I can’t find the link) that before long Trump won’t even bother to hide it: it’ll just be “Sure, we worked with the Russians to take down Crooked Hilary, so what?” Trump voters won’t mind (getting rid of the Evil She-Antichrist would be a plus for them), and Republicans in Congress won’t do anything.
Which is another depressing point: the problem isn’t just Trump, it’s The Republicans. It’d be easy to write off Trump as the problem so everyone can get back to thinking Republicans are still a functioning party that can be trusted to run the government, rather than dysfunctional extremists whose only goals are tax cuts for the rich, deregulation and playing to the white, male, Christian, straight supremacist part of the base (it’s a large part). As Echidne says, they aren’t fighting for principles, they’re fighting to win. It’s an end in itself. For a large part of the party, Democrats are no longer legitimate.
Part of that is, they don’t have a choice. Their agenda is hugely unpopular. The base is shrinking into one of many minorities in America rather than the dominant group. The base would have a hissy fit (or a lot of them would) if the party tried to reach out to Muslims (who voted overwhelmingly Republican in 2000) or Latinos because that would just drive home that this country no longer belongs to white people alone. Trying to function as a normal political party is not an option.
Case in point, Repubs are now denouncing the CBO for not saying what they want (facts, as the saying goes, have a liberal bias).