Why? He is wildly popular out there in deplorable land. The most voted for sitting president, what would give anyone the right to deny the people a choice that they want. Is it not a democracy? They should back off, they left it way too late and no matter what the truth is, it makes the USA look like a banana republic with incumbents using the legal system to lock their opponents up. The people KNOW what has gone down, yet he still has sufficient support, simply because ENOUGH Americans have misgivings about the whole episode.
If these folk were serious and earnest about their democracy, they would have focused on fixing the issues that led to the result being called into question in the first place. However, they aren't serious, the only thing they are serious about is having a system they can manipulate to their advantage. Face it, your system is broken and as it stands will probably never deliver a result that will look credible to the majority of voters. This is what should concern people like you, no matter what you believe, or you think you can prove, a massive chunk of the voting public are not convinced that the playing field is level. Fix that perception and the problem goes away, and you will never fix that perception by going after Trump. All you do is entrench the idea that they can't win in a fair fight, and that increases his popularity.
Whatever your wet dream is, the last thing US democracy needs is Trump sidelined. He needs to be beaten in a credible, fair fight by a solid candidate, not a basement dwelling Alzheimer's candidate.