Because a hard ending is better than a post-game in which the choices you made don't matter squat, like what happened with BS. No matter which of the four factions you choose, there would be radical change and the new situation would create enough things to do to justify a whole new game...a DLC would not pay it justice.
The worse thing is, I actually had a save before the final boss fight but I overwrote it
>:(
Why? On XBox you had 100 save slots, PC even more. It's funny how many people seem uninterested in taking advantage of them.