When discussing UU one has to mention that it had simply insane system requirements at the time. If I remember correctly it required a 386DX with at least 2MB RAM, which at the time of release meant a PC worth several thousand bucks. To boot, it was also a pretty expensive game. Wolfenstein OTOH ran fine on a 286 in fullscreen, AND it was shareware. That's why everyone and his dog played it, although its engine was less advanced than UU's. It was a very deliberate decision made by id/Carmack, and it payed off big time.