It was added after about a year or so, and there was two real driving forces behind it. The first was that people started using 'tags' in their submission titles to do things like "Ask Reddit" and also that the main page started getting overrun with Ron Paul tributes because apparently he was the second coming of Christ and would be the panacea that the US needed.

And since most of us started visiting Reddit for the tech/programming/science content people saw that stuff as spam.

Still, the subreddit system is really the only reason I still frequent Reddit. The smaller you go, the higher quality the content becomes and the better the community is (in general).