Because they hire people who clearly don't have the skills to be working on the ps3 platform.
I don't think it is a matter of skill, I think it's more one of motivation. I honestly think that if Bethesda wanted to make a game that worked properly on PS3 they could, the sad fact is, they just don't want to spend the time in doing so.
For me and many others though, the state of the game and the lack of DLC have become the secondary issue. After yesterday's pathetic show from GStaff on the blog (he flat out refused to answer questions relating to the exclusivity of Dragonborn or progress on PS3 DLC) it's the lies that surround the PS3 version of Skyrim that we're angry about. The claim to have learned from Fallout 3, the claim that the PS3 had received tons of attention and would on par with the 360 receiving the same level of support after launch. The claim that after Dawnguard failed to appear after 30 days that Bethesda have been forever "working on it" with zero progress made. The lies are mounting up and I think it's time for people to move away from the lack of DLC and state of the game and start challenging Bethesda on those lies.
Can't wait to see all of the articles pop up today, it's going to be awesome.