There are a LOT of corporate headquarters located in and around Houston, so there is obviously some added appeal there. If I had to take a wild-ass guess, I'd say that the cost of doing business in Texas is less than that in Michigan, due to Texas STATE policy. I don't think intrusion by the FEDERAL government, or any lack thereof, factored in heavily.

As for why nobody's moving to Detroit, well, cost of doing business aside, it's a $hithole. I'm more sure about this than my speculation regarding Houston's diversification.