While weather is a big issue, I am sure the NFL can give 100 reasons why Green Bay would not be considered. Off the top of my head; the age of their stadium and the lack of infrastructure to support it (transportation, restaraunts, hotel, etc.) come immediately to mind.
It is totally about the tourist dollars and the week-long festivities leading up to the game. The SB is more about what goes on before the game as much as what happens on game day now.
I wish it were in a freezing cold place...it would drive all the celebrities and posers away and leave the seats available to the true football fans.