I have a sort of theory about this. Left politics is theoretically about equality of distribution, so the idea of being rewarded for your support (even through influence) seems wrong. You do the right thing because it's the right thing to do! But … 1/2

… withholding support doesn't grant you any personal benefit. Your withdrawal leaves you unsullied, and — because you will in fact regret your candidate's loss — it can even seem morally virtuous. So right-wing candidates get the carrots, lefties only ever get the stick. 2/2