Sometimes, a developer or publisher chooses not to release a game on the PS3 for manpower or technical reasons (see: Valve). Other times, though - as is the case with Splinter Cell: Conviction - it's just down to money.
In an interview with Kikizo, Ubisoft's Steven Masters has completely gone against previous Ubisoft comments on the matter and been refreshingly frank, saying the real reason the game won't be appearing on Sony's console has nothing to do with his development team, and everything to do with the suits upstairs:
Ubisoft as a company now has a lot of experience with PS3. Our processes, tools, techniques are very well-developed - we could absolutely execute on the PS3 if we had the opportunity, but like I said it was a business decision.
A "link of heart" my arse. You know, in five year's time, I'd love to see just how much Microsoft spent on securing all these "exclusives".
Interview: Splinter Cell: Conviction [Kikizo]