Well, I think some of it has to do what else is going on. For one, the NFL isn't going to have a major event the same weekend as the Final Four, and they're not going to go against the Masters. For another, and I think this may be more important, is that most schools don't end classes until the end of April. With teams wanting to hold minicamps right after the draft, players don't - or can't - come until their classes are done. It seems like there are a handfull of players that fall into that category every year, and I would think the teams want to minimize that.
Plus, I think teams want to get to it, but they also want as much time as they can to evaluate and reevaluate all these guys that are in the draft. To me, it's like taking a board exam. You want as much time as you can to prepare for it.
That said, I wish they'd move it up, too. I hate waiting till the end of April.