It's no surprise really. Games like Quake only appealed to the more hardcore, nerdy gamer back in the 90's and as such catered for them. These days gaming is targeted at near enough everyone and as such the playing field has been leveled so even the most inept player will be able to play with out too much trouble. Why make a hard to play game when you can make an interactive movie that almost plays itself? It's clear which one will sell and make millions.
I played the expansion packs for the first time the other day. Completely forgot they existed. I was surprised at how hard they were.