Because the founders assumed that our rulers would be rational actors. They didn't have a plan for an entire party going bad. "Oh, sure, we'll just add an impeachment clause so nobody has to get assassinated..." (paraphrasing Ben Franklin):
"The Founding Fathers wrote impeachment—originally a Roman political institution—into the constitution for the purpose of removing an official who had “rendered himself obnoxious,” in the words of Benjamin Franklin. Without impeachment, Franklin argued, citizens’ only recourse was assassination, which would leave the political official “not only deprived of his life but of the opportunity of vindicating his character.”
But they didn't take the extra step to realize "What happens if that party refuses to impeach? Or indict? Or convict?" Suddenly there is no alternative.
Ideally, there needs to be a process to nullify the entire party, but I have no idea how you'd go about it.