There are many reasons why college basketball is popular, but none of them are as important as this: The best team doesn't always win.
Face it, whatever reason you cite for watching the game, you wouldn't care so much if the game were a true test of superiority. Some of the sport's most famous games are ones in which the better team lost, and the zaniness that is March was built on a foundation of unheralded teams pulling out victories when they weren't given a chance.
The reasons the best team doesn't always win are varied, but they can be grouped into two categories. The first is what I'll call "consistency," or the natural variation of a team's play. Some days a great team is clicking on all cylinders and truly is unbeatable, but there are a few games during the season in which every team underachieves. Consistency is important in determining how often the better team wins; a great team that is inconsistent is more vulnerable to an upset.