It is because Google + is their new identity service. Everything requires it, instead of a gmail account. Google + is your Google identity. At the same time it is also a social platform because so what? Don't use it. Or, use it since it's really really useful.

The amount of rage people express about being "forced" to use Google + is as perplexing to Google as it is to me. It's an identity service.

You don't need to have a gmail account to use Google's services anymore you just need a Google identity. That's what Google + is. I don't know how many times I can say it but so far it seems like a lot of times. People want to rage.