He was going through a tremendous change in his life. Moving from Cuba, learning English, having more money than he knew what to do with is harder than you guys give it credit for. To keep the transition comfortable, the left him at 2b until they felt his transition was completed smoothly. I can't fault them for this in the least. Once he's comfortable and hitting, then you can mess around with where he's playing. It hasn't hindered his development and he wouldn't have been up any sooner anyway. He'll have all offseason to workout at 3b.
Similarly, didn't they leave Betts at 2b for awhile in the minors? That seemed to work out ok.