Those three games are not equal. Planescape: Torment did not sell badly; it did alright, in fact. Sure, it wasn't a huge success like Baldur's Gate, but the game did not do badly. It is true that those other two's sales were disappointing, though; though Lucasarts did claim that Grim Fandango "met expectations" in the US and exceeded them worldwide, it was not exactly a hit, sadly enough, and System Shock 2 does seem to actually have sold badly (the only one of the three that was a clear failure).