Rémi (the author of goratings) posted some reasons on L19 forum (which is currently down but would be up again soon hopefully). There's also a discussion on reddit (https://www.reddit.com/r/baduk/comments/5r5wxy/goratingsorg_remove_alphago_and_deepzengo_from/)
Generally, part of the reason is that (quoting uberdude) the approach Rémi is actually taking is that different versions of bots have big differences in strength, in contrast to the more gradual improvement/decline of human players which is assumed in the Whole History Rating model, so it doesn't make sense to include them as a single player.
