I dont know why more companies dont copy Bethesda and make quality RPG's for the PS3. Is it because they dont know how? Don't have the talent? They would make sooooo much money. I know the PS3 isn't as technologically powerful as a PC, but I would be more than happy with the PS3's Oblivion graphics and map size.
Large RPGs are very difficult and time-consuming to make. Even with two teams there are 2-3 years between Bethesda releases, which means generally 5-6 years between titles in a series. On other other hand a successful FPS can often upgrade the engine and gameplay and release a new iteration faster and far cheaper.
Since they take more time and are more expensive to make they're much riskier ventures, so it's hard to break into. Sandbox games have become quite popular in the past few years, perhaps one of the developers working on them (like svcker Punch) will try a sandbox RPG in the future.