State-level political polarisation

Andrew Gelman points to some really neat data work by a colleague of his, Boris Shor (jointly with Nolan McCarty) on ideological positions of US State legislators. How does he put them on a common scale? By exploiting that many members of State assemblies go on to Congress where they then get an ADA score. So long as the state legislator's ideology doesn't change in the move from state legislature to Congress (or, more importantly, doesn't exhibit any changes that are specific to any legislators coming from that state), we then have some fixed reference points in each state legislature.

Gelman provides this picture from Shor:

Some things aren't that interesting to me. Of course New York, Connecticut and Maine have both Democrats and Republicans far to the left of the Congressional averages. What's more interesting is the gaps between the parties. In Utah, Indiana, Wisconsin and California (and a couple others), there is no overlap between the Democratic range and the Republican range. In Rhode Island, there's no overlap but only a tiny difference in means. In lots of other states, there's substantial overlap between the two parties' positions. I wonder what drives that kind of state by state variation. Open versus closed primaries? Something else?