Because they're adding them on their online subscription service.
Other people do?
It takes time to convert and test the games too, don't forget that. I have no doubt we'll be seeing the other consoles added as time passes
This is what we've been asking for, a Netflix style subscription to play classic games (with added features) and now we've got it, we're still complaining because they're not dumping everything in one go with no care for quality control.