It's an electoral system issue. Voting is voluntary, which causes highly-motivated single-issue voters to be overrepresented. Then there's the primary system, w

It's an electoral system issue. Voting is voluntary, which causes highly-motivated single-issue voters to be overrepresented. Then there's the primary system, which is the same problem squared. It used to be that politicians could pander a little to the base during a primary and then "run to the centre" in the general. Those days are gone, because "being primaried" is a real and persistent risk. The practical upshot is that the system is structured to favour fringe voices rather than the median voter.