I think it's similar to Netflix replacing stars with thumbs up/down. They realised they didn't need that data, they just needed more dimensions (ie for you to rate more of their library).
Slashdot didn't need to limit voting so severely. And it turns out replies to funny comments are usually funny and interesting comments are usually interesting.
They also never scaled it - +5 was the maximum from when an article might top out at 200 comments or 5000 comments.