Probably because Epic has never really invested any effort into optimizing it, concentrating on just UE3 instead; and the Bioshock devs don't have the resources to do it instead of them.
UE3 games seem to be okay on the PS3 nowadays but Bioshock's on a much older version.
UE3 games seem to be okay on the PS3 nowadays but Bioshock's on a much older version.