By the way, downloads of source aren't going away (i.e., get a zip or tar ball)! Just the "upload anything and make it available" feature. There seems to be a little confusion about that.
They probably determined the excess bloat and strain on their infrastructure from git cloning huge binaries outweighed the benefits so they are removing the feature.
Which, conveniently enough, is exactly what they want us to do ourselves now. I already pay GitHub to do that for me, how is it a benefit for me, the paying customer, to do that myself by hand?