Probably for similar reasons why you need a commercial pilot license. If you're going to be doing it for money, you should have more training than someone just going to wal-mart and picking up a drone. However, as such a training regimen has not been defined yet, the FAA has decided that it's safer just to say it's off limits for now.

Personally, I'd love to see more effort into setting up such a commercial program, as there are a lot of dangerous jobs that could be made safer through drones. For example, there are dozens of crop duster accidents each year where someone's injured or killed. Pulling someone out of the pilot seat, and onto the ground can make that job a lot safer and more accurate. But, we can't get there until we do the studies to figure out how to do such safely.