User blog comment:The Milkman/Twitter Responses: Bethesda Still Having Issues/@comment-5501398-20121014042649/@comment-3492791-20121014055922

The issue comes down to software porting.

The PS3 uses very different memory allocation to other platforms. Thus large sections of the code can not be transferred and have to be rewritten. This largely rewritten code means that to add Dawnguard to Skyrim, it will have to be a very different Dawnguard to what PC and 360 have. This likely comes down to some portions of the code being purely incompatible. If they wrong data goes to the wrong place, it isn't able to accept it and crashes the program.