I think it's because of the interface. It looks like Google Plus is less user friendly. There are less options, and no games since 2013. People often do not reply to questions on their Google account, and many companies do not use it at all. At least, the sharing apps seem to originate from Google to other social networks and not the other way around. But is that a good thing? Maybe there would be more users on Google if they permitted other networks to repost on Google. Do you know examples where Google Plus wins over the other social networks?