One word that sums it up:
Money
More people bet on the NCAA Men's Basketball Tournament than for any NBA playoff series. Even people that really don't gamble get in on the action.
The colleges see more revenue by way of their share of the NCAA
TV contract. They attract students because of their success on the court; they attract fans from all over who'll gladly fork over $49.99 for a team sweatshirt.
The top college players are playing for money down the road, so they want to be successful in the tourney so they can get drafted by the NBA and make the money. Unfortunately, this is usually without a college degree.....
Money is also what has ruined pro sports. But that is for another thread.
In the NCAA, you don't see a bunch of spoiled rich kids whining over their playing time and bitching about the coach; You have players that are playing because they know that they'll never be good enough to make it on the professional level, so they go out and leave it all out on the court.
And for a final reason:
Bobby Knight. Watching a Bobby Knight-coached team is like watching a car race waiting for a crash, you just wondering when Booby's going to have a meltdown (Same was true in the NFL watching Mike Ditka on the sidelines.). Will he throw a chair at the refs or will he just cuss out his players?