Well part of the problem is that Sony doesn't have too many first part titles, at least not in comparison to MS and Nintendo, they rely heavily on 3rd parties for most of their exclusives and 3rd parties just follow the money, there's no incentive for most of them to keep their games Sony exclusive.
At least they've got the GT, God of War, and ICO franchises
It seems they're attempting to revive some older franchises too like Warhawk
I'd like to see them revive Wipeout and Parappa personally.
Despite the franchises they DO have they've been squandering them to keep the PS2 business afloat leaving the PS3 without even strong support from Sony themselves.
That's not to say that titles like Motorstorm or Dark Legonds are bad (ok, maybe some of them are) but they're not exactly the kind of titles that get people to go out and spend $600 on a console. For that you need a Halo, FF, Zelda, etc. At best titles like Motorstorm are filler to round out the line up... not the killer app.
Meanwhile if they had made God of War II for the PS3 I guarantee they would have sold a hell of a lot more PS3s.