It boils down to PS3. It's very expensive and time consuming to release a patch on the PS3. Each client update must go through Format at SCEE and SCEA and neither of them have very fast turn around. I've seen updates take as long as 2 months to get approved (or denied) after submission. Then, if the update fails Format QA, then they start the process all over again.

That's why you dont see a new update coming out every month or two.