The reason the PS3 has a lower attach rate is because of the price tag. After you buy it, you have less money to spend on games. The 360 isn't cheap either, but it benefits from a very strong internet community and still being affordable. This is part of the reason the Wii has a higher attach rate than the PS3 despite more casual gamers. And it's also part of the reason why the Gamecube had such a high attach rate.
Even if it's not the core reason, the logic is undeniable. The more expensive the system, the less money you have for games.







