basically its too much work, and focus testing showed that console gamers enjoyed swords axes and bows the most, so that's all they put in the game.
I'm not outraged; It's pretty understandable really. All the money comes from console sales and I wouldn't expect very many people who use consoles to be able to enjoy anything if it were as complex and great as Morrowind because they'd actually have to make a decision from time to time, or think about something.
Still, they could have added in some extra, un-used mechanics or whatever that would make adding in throwing weapons and crossbows viable for modders for the PC users who actually enjoy diversity and challenge, so that in the future we could turn this game in to an actual PC game where each weapon has advantages and disadvantages instead of all being the same.