Certain games don't see a demo until months or even a year later. Remember when Call of Duty: World at War (I think) had the demo added to the store a year after launch?
Ratchet and Clank demos I believe weren't prepared until about a month after the game was in stores in the past. This tactic would give gamers who haven't already bought the game a chance to see what they're missing. Of course, it would also send the first ones into the fray blind.
Then again, some demos for games release the same week as the game does. It would help more if companies let gamers try it before they buy it, but there could be multiple reasons for why they won't... from knowing it'll sell anyway to perhaps if we know it stinks then it's one more to avoid.