UST, if you count grad students, is much larger than any other school in the MIAC. The one reason I could see them not moving up to D2, is that, at least here in MN, the MIAC gets a lot more media coverage than any of the D2 schools in the state, for whatever reason.
Having attended a D2 school myself, I can tell you that nobody cared one whit about any of the sports programs, and I've heard it to be true at most of the other state universities, with the exception of those with D1 hockey.
On the flip side, I have a few friends who went to MIAC schools, and sports were a big deal at most of them and still are. I've often thought it weird that Concordia-St. Paul decided to move up to D2, rather than try and move from the UMAC to the MIAC, where they'd be a better fit. (that and changing their name from Comets, which was cool, to Golden Bears, which is lame.)
One thing quite different about Wisconsin, is almost all the UW schools except Green Bay, Madison and Milwaukee are D3. There's only one D2 school in the state - UW Parkside. I wonder if that saves the state some money.