Backs usually leave early because they either 1) are sure fire bets to make money right now, or 2) want to hurry up and get going on what will be a short shelf life in the NFL. Boom is not a sure bet to make a roster as of today. His speed and explosiveness is tantalizing, but he's has the injury bug, doesn't catch the ball well, is inconsistent (but better than he was) as a blocker, and doesn't run through tackles in the interior.
Now, as for whether or not he wants to get the clock moving, that's another story. If he believes he can't improve himself--and thus his stock--next year, he's gone. However, for a guy with a bunch of yards, he has surprisingly little carries on his body. Snell will have more career carries at the end of his sophomore year than Boom will have at the end of this year. Part of that is Booms lack of durability (nicked up all the time or just plain injured) part of that is stamina and always sharing carries. One could make the case that Boom should roll the dice and stay, just to see if he can turn in a huge year and really jump up boards.