The primary reason was that I really enjoyed the flexibility of Michigan's program over Columbia's. Columbia's seemed very rigid and that's just not something I enjoy in a program. I want to be able to take classes specific to my career path rather than a very specific list of classes.
It seemed much easier to get a GSI or research position at Michigan. This is just a perception.
In talking to students, those at Michigan just seemed much more enthusiastic and happy with their program than Columbia (this was just a personal observation).
And, to be honest, the idea of living in NYC terrified me.
Over the ones in the south, it was purely a money decision. I think it's incredibly unwise to take out many more loans than necessary. Schools in the south offered me nothing but loans whereas Michigan and Columbia offered me scholarships.
Over every school though, the flexibility of Michigan's curriculum is what attracted me.