There are a ton of reasons for prospects to skip the combines. It would seem logical for a prospect to want to go and to show off their skills versus others but with the advent of draft sites, mock drafts, measurements at schools, measurements at basketball camps, and the fact that just about every prospect has video of him playing since he was in middle school, the combines aren't always going to be beneficial for every prospect. When selling ANYTHING, and that's what prospects do by going to the combine, they are selling themselves, too much information isn't always good.
All the information that is readily available to us, is also readily available to the NBA team's scouting departments and they also have so much more. The teams that are interested in a prospect that the team feels will be available to them, will bring the player in to interview him, measure him and work him out. That's a lot more crucial event for the prospects than the combine.