I'm guessing most of the empty seats are corporate - so the employees of said companies never paid for the tickets. The only way to fill these unused seats is to sell SRO's and let people take these seats during the games (a better way would be to have the companies themselves surrender the unused tickets as punishment for leaving their seats unoccupied repeatedly).
As far as SRO goes, it's a ridiculous deal since the cost of these tickets is only a couple bucks cheaper than assigned seating is (at least it was the case when the U actually offered single game tickets for assigned seating a few years ago - I believe now the only tickets offered to the public are SRO). So basically to get in, you either have to buy on the street or go the SRO route - and then grab an empty seat a few minutes after the game starts and run the risk of being booted out when the actual seat holder shows up (since people come in notoriously late to Gopher games).
Beyond that, getting season tickets is a costly and difficult process. Either you go on the wait list (which is at least 2000 people deep I think) which drops only a dozen or so per season, or you go the donation route. The latter approach is really the only way to get season tickets anytime this century, and it requires a minimum donation of $500(?) per season on top of the cost of the ticket. I don't know about the rest of you, but forking over what amounts to $50 per game to see college hockey is *'ing ridiculous. Add in the time it takes to drive to / from campus in heavy traffic (particularly on Fridays) and the $10 it costs to park, and this pretty much becomes a no-brainer for most people. Why pay $60 per game + gas/travel time when you can watch it on a big screen in the comfort of your home on FSN?
Then there's the fact I could spend the same amount of money and go to a Wild game instead and see hockey at the highest level. If I'm going to blow $50+ per game, that route makes a lot more sense than going to Mariucci, doesn't it?