Fallout 1/2, PST, the Baldur's gates and so on have adapted to their environment and are still ruled as kings among games, making them immortal, like the Tyrannosaurus Rex or the Megalodon. These later kinds of games, namely Mass Effect and Oblivion and alike, are just flukes of evolution that has failed to adapted themselves to their environment. Fallout 3 will join them as Brotherhood steel, BD: Dark Alliance and sink into the ranks of fossils, only to be brought up once in a while by only the most insane archeologist.
Afterall, just like in nature, 95% of all species(games) have been eradicated through the years.