Someone mentioned demographics... I think there's a noticeable change there.
Not only that, but... (OK, this is more from a console aspect) do you remember mascots, usually found in kids games? Like Crash, Spyro, Jersey Devil, Klonoa, Jazz Jackrabbit, Gex... Kids today are fed games like GTA and CoD, hell, kids today are playing God of War III because "I wuv to wip teh medoozys head!"
There are so many games on the market today that it's hard to follow for the avreage consumer and so they go for the most hyped ones - parents usually have no idea what game to buy for their kids, while teens are already on the popularity bandwagon and they can't be caught playing a "kiddy" game like Braid. This all leads to a sort of brainwashed adult state where the average gamer is used to SHOOTAN LOOTAN KILLAN.
Another thing which I believe helps this 'manipulation' is accessibility. Beyond the usual easy-normal-hard-ultrahard difficulty options - gaming is noticeably dumbed down. What are the odds of having a new and original, but genuinely challenging, game? The only real challenge I get from games today is right here on GOG's classics and some rare new gems of the indie community. The best example of dumbed-down I can provide currently is Darksiders - a mix of every popular game on the market, not just in gameplay, but in artistic 'vision' as well - pretty, but no brain activity needed.
I don't see a problem with 'casual' gaming though. Tetris is casual, Pac-man is casual, even Super Mario is somewhat casual. Most 'ordinary' people are satisifed with such simple games - just like some people like to watch sit-coms all day, but don't prefer seeing the latest blockbuster hit in cinemas. Sometimes, casual gaming could even lead to further exploration of the medium (like a friend of mine who started on Farmville and ended up on Ben There, Dan That!)
tl;dr Gaming is bigger, crap is bigger. More quantity, less quality. Less quality - less customer taste. Less taste - easier money.