When you need every vote to get legislature to pass, because you control 51% of a chamber, backbenchers on the ideological fringe of a party, (DINOs and RINOs) have a lot of power.

When you have a majority with comfortable margins, you can care a lot less about what the Sinemas and Manchins and McCains of a party think.

There are many reasons for why two-party FPTP sucks, but this phenomena is present in multi-party systems, too. And, of course, sometimes politicians end up crossing the aisle, much to the chagrin of the party whip.

"In fact, [proportional representation] robs him of personal responsibility; it makes of him a voting machine rather than a thinking and feeling person. In my view, this is by itself a sufficient argument against proportional representation. For what we need in politics are individuals who can judge on their own and who are prepared to carry personal responsibility."

https://www.economist.com/democracy-in-america/2016/01/31/fr...