I can see both sides of the discussion.
Yes, a Spring Game played outdoors in front of a fairly large crowd definitely does have more of the feel of an 'actual' game.
the unanswered question is this: if the weather had been nice and the Gopher game was held outdoors, how many people would have shown up? In previous years, the attendance at outdoor Gopher Spring Games has never been what you could call "large."
it's a cultural thing - the culture of Nebraska football traditionally includes bigger crowds at the Spring Game. And they have a new head coach, which I think would increase curiosity about what the team will look like.
bottom line - last year Nebraska reported average attendance at home games of just over 86,000. Minnesota reported average attendance of just over 45,000.
Nebraska football has more fans than Minnesota football. Their team may suck, but their fans show up.