Mainly to stop people trying to game the system by upvoting everything. We currently have over 6,000 questions and 13,000 answers. In theory you could get nearly 20,000 points just by upvoting EVERYTHING.
Of course, that takes a helluva long time but some people did manage to get a few hundred points quickly by upvoting a lot. So I removed the point for upvoting. You still lose 2 points for downvoting, this is to discourage random or spammy down voting. (And as mentioned many times before, if the post is spam or just plain wrong it will likely get hidden and you may be those points back.)
Also I feel you shouldn't be "rewarded" for just upvoting, it's something everyone should do where appropriate. Hope that all makes sense!