Over the last 20-odd years, I have gone from a rabid Vikings fan and a casual Gopher fan to a rabid Gopher fan and a more casual Vikings fan. I did not watch the Vikes' season opener - was covering a HS volleyball match, and didn't bother to DVR the game. That never would have happened a few years ago.
I agree that someone can be a fan of both teams. But, I suspect that most people are weighted toward one team or the other. It may be 60% Vikes to 40% Gophers, or some other ratio (not the Randy Ratio), but I suspect that most fans are not a 50/50 split.
If the Vikes have had more attention and more devoted fans, I would think that part of it comes from the NFL's marketing genius. The NFL is a big national story almost year-round. The Draft - OTA's, mini-camps, training camp, pre-season, regular season, post-season, preparing for the Draft......it never ends. No comparison between the NFL and College FB when it comes to marketing and promotion.
And - one of the biggest things going for the NFL - Fantasy Football. I don't get it, personally, but I know people who are just bat-bleep crazy for Fantasy Football. I knew a guy who was in 4 leagues, and commissioner of 2 leagues simultaneously. Let's just say that his work performance suffered a little bit during FB season, especially on Monday.