Because he was the same age, same height, much more athletic, much stronger, in much better shape, and had just lead the Big 10 in scoring.
Jefferson was fat, had no college experience, slow, and unathletic.
Jefferson averaged like 40,20, and 8 in HS. The Jazz just didn't want to draft the HS player. Jefferson clearly had more potential (because you can obviously see Jefferson is the better player, that is not a product of being developed, he just had more ability).