If he's not ready, one more year of development in the NCAA maybe preferable to a little time in the NHL and then the bulk of time in the AHL. If he needs that extra development time, it can allow him to hit the NHL the following year and never look back, which sets him up for a higher-earning, 8-year contract coming off his entry level deal, as opposed to needing some bridge deal and waiting another 2 years to sign the big deal.
Additionally, NCAA schedule is such that if he needs to physically bulk up some more, it allows for that. Otherwise you have him needing to add muscle, but also trying to compete for a spot on the roster. That can be a difficult situation to maneuver in and why you sometimes see guys really bulk up in the offseason. This might be a more ideal development curve.