TWFIrst is right, back a few years ago when NW was going down the toilet, Minnesota bailed them out big time. Plus, there are other reasons I read the stay in MSP. I read a article in the Wall Street Journal about it.
Plus, even though people rave abou the New DTW 1.2 billion terminal, well remeber, over the last 15 years, NW has put Nearly 7 billion into MSP, that was probably the reason it go rated the best airport in north america by the IATA and Travel and Liesure. Plus, NWA has a huge stronghold on MSP, almost monopolistic. Anytime a new airline tries compeating, the kill it (sun Country). Every wonder why BA, LH and AF don't fly into MSP? Or why now that BMI applied to fly here, it is having so much trouble geting in? It has nothing to do with the market here, theres plenty.
Also, DTW is just slightly larger than MSP. MSP beat it out in Pax or movements, i don't remeber which one.