They do it because:
A) People are going to buy the game anyways (you will, suck it up)
B) Blizzard is obsessed with dated GFX for a few reasons; #1 is money and #2 because it runs on all PCs which brings it back to #1
Just my opinion but that's the way I feel.
I'm looking forward to Skyrim first anyways, not worrying about D3
