But they aren't better represented unless everyone else does the same.

Suppose California were to do it, resulting in a proportional allocation of seats in the House for its delegation. If this causes the House to swing from Democratic majority to Republican majority, the net effect is the opposite of what most Californians wanted.

Don't get me wrong, I get the point that it is a more fair and equitable way of doing things, and in principle, I agree. But if you play fair at a table where everybody else cheats, you lose. My state (WA) also has referendums, and if such a proposal would come up, I would absolutely vote against it - unless it was some kind of interstate compact where another similarly-sized red state were to implement the same reform at the same time.

I’ve had this exact thought: that Texas and California should have some sort of compact to do it at the same time. That would be a boon for Texas Democrats (of whom there are many) and California Republicans (ditto).