On a per-case basis, some developers see the effort of porting to PS3 as too costly, notably because of the tech requirements and you can't just take your engine over.You are contradicting yourself within your own post. If concentrating on a single platform isn't a good business model then how can it not be worth it to migrate to the PS3 even though its not DirectX based.
An opinion I have shared before. Some Insomniac content is remaining PS3 only (they have a PS3 engine and don't care to refactor it for a market they don't think exists in sufficient numbers outside of the PS3 platform, I guess). Overall though, previously platform-exclusive developers are going multiplatform, and the reasons for that are economic, nothing else.Whether or not, exclusivity makes sense is dependent on individual circumstances.
But the economics of exclusivity clearly don't favour it, because every 3rd party has gone cross-platform. Final Fantasy, anyone?! Seriously, it's plain to see, everyone and their dog is wanting to take their games across platforms and not be specific to one console, except in a few fringe cases.Furthermore, being exclusive creates a lot of indirect marketing for titles...
Insomniac isn't a small developer. Of course for small developers it makes sense, hence lots of exclusive content on the DLC platforms.Plus, for small developers with limited resources it makes sense to go exclusive.