It’s not rocket science. The only publicly said reason it’s never happened before is they want to respect the decision of their constituents. Unofficially, nobody wanted to see themselves on that chopping block.
From a political scientist game theory point of view it’s possible that a majority faction of party A throws a minority faction of Party A on the chopping block and Party B obliges them because they think they can pick up those voters.
So yeah, a lot of resistance to actually unseating someone. For good and bad reasons.