Because politicians need to be held accountable for their inaction. Blaming voters is exactly what political donors and lobbyists want because it means they don't have to bear responsibility for the candidates they back, and it gives politicians a convenient out since they can just continue to blame the other party while they sit back, attend fundraisers, and pad their pockets.
Besides, we live in a heavily gerrymandered society where more than 80% of Congressional seats are in "safe" districts where one party is virtually guaranteed to win every time. You can vote your conscience all the live-long day, but it won't make a difference if you live in a place where your vote quite literally doesn't matter
