The problem is the odds of Michigan playing in a bowl game on New Year's Day are high (all it takes is a top 5 finish in the Big Ten) and a hockey game at the Big House airing concurrently creates a real conflict both for TV viewers and for the university itself. I'm also unsure if a Winter Classic, if not against a traditional eastern/great lakes team, could hope to match the record the Big Chill set. It would be a bit depressing putting the thing on in the country's biggest stadium and then not selling it out.
What Comerica lacks in size and je ne sais quoi it more than makes up for in ease and, this is crucial, location. Getting anywhere in Metro Detroit during the winter is a chore, but going downtown is still a significantly easier drive for many people than going to Ann Arbor. Granted, it looks likely that commuter rail will be in place by then and that would serve a lot of people in the Wings western Wayne heartland, but that's still cutting off a lot of fans.