I blame Sony.
They had a year to see how a meager 512MB of RAM worked in the Xbox 360, then decided to launch anyway (five hundred and ninety nine US dollars) with not only the same amount of RAM, but with it split up in such a way (256MB system RAM, 256MB VRAM versus 360's 512MB of RAM for any purpose) that developers can make great-<b>looking</b> games, but not necessarily complex games with much in the way of large data sets.
I suppose Bethesda could have changed their design on ALL platforms to better fit the PS3's limitations, and I think that's pretty much why the best-selling PS3 games are the exclusives (they've been designed around the system's limitations), where as many of the best-selling 360 games are multiplatform titles. But why should Bethesda do that? They're not the ones putting so little (and so inflexible) RAM into a system with the intent of making it a flagship console for seven (or more) years.
Maybe it'd have been better if Bethesda just didn't put these games out on the PS3 at all.