This is something that's puzzled me as well. I don't know why more states haven't moved towards a unicameral legislature, especially considering the vast majority of state legislatures are under one-party rule. That doesn't even take into account the executive. Most states are under one-party rule overall, whether that includes one party controlling the entire state government or one party with a legislative supermajority. In those cases, especially, it seems like they would work far better under a parliamentary system. Personally, I like the efficiency and accountability that it could bring. I think the only reason things are as they are is because of habit. If a state were to draw up a completely new constitution from scratch, I couldn't imagine a reasonable proposal being a bicameral legislature with a separately-elected executive.
Legislators aren't keen to put themselves out of a job or cut off career paths...
