That's due to politics being a team sport, and everyone, including the voters, understanding that it's a team sport.
Getting your team control of a branch of government is way more important than having a 'good' rep in your district, because if you don't, they won't have any ability to do anything for it anyways.
If you couldn't get someone you wanted in the primaries, you just have to hold your nose, close your eyes, lie back, and vote for whomever made it through.
Whether this results in long term problems is a bit of an academic question, given that every election in the past decade is one where you either get to vote for the status quo, or an insane cult of personality.