Because to make a profit, you'll first have to break even and I don't see that happening with the insane developing costs this game swallowed over the years.
Furthermore I don't think there's much of a fanbase for this game. Had it not been the subject of constant jokes and had it not been popular for the sole reason of not existing, people could probably care less about this game.
DNF is going to be the typical sort of game no-one really wants to spend any money on, at least not full retail price, but does want to see how it turned out. So I predict that the illegal download numbers are going to exceed the actual sales, and not real profit of any kind is going to be made.