I don't remember if it was last year or the year before, but I do remember reading an article the talked about this and a study done just a few years before that showed that few college players drafted as a Jr. ever returned to college. And as one might expect, those drafted earlier you find even fewer. Also, very few MLB player ever finish college.
Yeah, you'd thing that with only 1 remaining year of school (and particularly if the player's contract has a provision for that year being completely paid for) that it would be easy enough to finish.
But then. . . .life does have a way of getting in the way of such plans. 