Because people don't want to have to go back to the store to get what they need. They made so many versions to cater to all the different groups (Even though most any tech savvy user would acquire Ultimate anyways). Pro and Home were too variant, there was no real "Business" edition, and people want very different things.
I agree however, that there are too many. We could do with just Home, Business, Ultimate, and Starter (Which you forgot, btw). I imagine that there is no real need for starter or Home to be 64 bit, so that would cut down as well. That would be best case scenario.
But god know, MS doesn't listen to us. They just care about money, and flashy effects, and turn their noses when we complain about bloat and stupid requirements.