Great question. and fundamentally because of the Constitution KG. It's held in semi-religious awe.
If you believe that the Russians or other actors have targeted and possibly tainted the US elections, but the President has not taken "care to administer the law" by actively encouraging that activity, or passively ignoring that activity, the remedy described in the Constitution is Impeachment of the President (and / or other Office holders) for "High Crimes and Misdemeanors". Under that process, the House of Representatives would investigate, and Impeach (similar to a Grand Jury investigating and Indicting) and the Senate would hold a trial. If 2/3 of the Senate vote aye, then the President is removed from office, the Vice President takes over etc.
I suspect that no court in the US would agree that a law that invalidated a presidential election and re-ran it was constitutional. I know that one court could be found that would say it was not, so that would need to go to the Supreme Court and that's been packed with Republican appointees. Re-runs do happen on a local level sometimes, and has happened in South Carolina recently because of documented cheating, but for such a law to be passed for a Presidential Election it would have to be passed by both Houses of Congress and I can't imagine the Republican controlled Senate would let it pass, and the President could veto it if they did. As overcoming that veto would need 2/3 of the Senators to vote for it - it's the same vote hurdle as Impeachment.
So Impeachment it is, or just vote him out in 2020. Either one relies on winning public sentiment, and that won't be easy given right wing media propaganda. I suspect that this issue will turn, as usual, on influencing specific States and Constituencies of Republican Senators and Representatives who are up for re-election in 2020 - will they see more personal advantage in voting for impeachment or backing up the President? All politicians are cowards at heart, so probably the best strategy is to push for more regulations around voting and make sure potential voters see Republicans arguing against that and getting them on the record as voting against that. The problem is the Republicans are already heavily involved in voter suppression anyway (not a partisan comment, it's plainly true) so will the voters see this as a just more partisan sniping or a genuine repair of democracy issue.
I wish I had confidence in the PR capabilities of Democrats to win that argument. The parallels are there for the UK too, how do you shut down the propaganda and bad faith of the right wing, and take action to protect what we have assumed to be shared democratic ideals and activities when there are actors only interested in power.
My fear is this is a re-run of Germany in the 1930's.