Because we have separation of powers. Unless you're just talking about merging the two houses of the legislative branch (which is just fine, makes things less clogged up anyway), it wouldn't work under state constitutions. As Sanchez said, our system really isn't going to change anytime soon. With every passing moment it is increasingly unlikely (due to interest groups, polarization) that any change in the structure of government will occur.
Separation of powers at the state level is only in place because of current state constitutions, it's not a requirement.
