Adventure games grew less popular as gaming became more mainstream. The majority of people favor less cerebral entertainment.
True. For me personally, adventure games died when DOOM came out. My friends and I all played the King's Quest/Space Quest/Police Quest/Quest for Glory series, etc. Then one day I'm walking by a computer booth showing off a DOOM gameplay loop and my jaw hit the floor.
Why walk around a static backdrop when I could grab a double-barreled shotgun and cause havoc in the underworld!?
Ironically though, FPS game have become static now and I long for game with a good story and gameplay more than anything...