Because they have no clue what they are doing. They think they can run a business through trial and error. They do stuff just to be doing it and see what happens.
In this case they want to try an ideal world full of butterflies and unicorns, and everyone agrees with everyone else. This is one of the most full of shit sites on the internet. But I guess some people want to pretend that all the bullshit posted here is legitimate. Then all the young gullible readers see it and think "Oh, lots of up votes and no down votes. So the moon really is made of cheese."
There's nothing logical about the design decisions this site makes, never has been. They don't beta test anything or respond to criticism, they just make layout changes on a whim, often creating "solutions" to non existent problems. For a site of its scale, it's the least user friendly site I've ever seen.
The same reason I think Youtube and Disqus did it, to stop upvote/downvote wars. But both of those sites/services still have the downvote symbol for comments, but when you click on it, it does absolutely nothing anymore.
Just to fuck with us by introducing more pointless updates or more likely, to provide an even more pc-friendly environment. Seeing as how a large part of GaG's appeal seems to be providing a way for everyone to get their ego-stroked.
There were multiple reasons for this change. One was simply because of the introduction of the like feature for questions and myTakes, it makes sense to extend that feature to opinions also in order to be consistent. However, we do also want to build more of a positive community atmosphere which was another reason for it. :)