Zweiguys
Winger
One reason for the Vikings stadium being covered is they can now, and did, host a Super Bowl game.A number of things. Tradition. Machismo. But probably most of all, and this is particularly true of the biggest stadiums in the north of the US, for engineering and costs reasons.
Such can be the weight of snow, it was often too expensive – and still is – to construct roofs that wouldn't collapse. You can also make these big roofless bowls bigger too, to fit more in, and, as a result, get more revenue.
Generally, the biggest football stadiums are actually in the college game rather than NFL and they are almost entirely huge, roofless bowls. If you can get 100-120k plus fans inside I guess you will, won't you?
Who cares that fans get snowed on, especially as NFL games are routinely played in very heavy snow?
There's also a chance to use the weather and stadium to your advantage for teams like Green Bay when the likes of the LA Chargers or Miami Dolphins come to town and feel that icy blast as they run out onto the field.
However, it's changing. NFL teams like the Vikings made the decision to build new stadiums that were entirely covered, making them all-year round facilities capable of holding concerts and things like the college basketball play-offs. You'll see less and less open bowls in the northern cities in future, I expect.
Until then a Super Bowl north of the Mason-Dixon Line was rare. Seattle vs Denver at the Meadowlands is the only other I can think of at the moment.