iMessage is a huge lock-in for non-technical users. They are just obsessed with it on iOS. You can find tons of forum posts with people throwing fits that XYZ Android phone doesn't have iMessage.
The reason people throw fits is because the experience between a group messaging together on iMessage is exceptional - this experience breaks down when even one of your friends in the chat doesn't have an Apple product. They aren't able to send or receive the majority of the "chat add ons" iMessage provides. I'm sure making the bubbles green vs. blue only helps to stoke the "us vs. them" fire.
I consider myself to be a reasonably technical user and still prefer to message with iMessage since I know the experience will be the same for everyone I'm chatting with. Yes, we _could_ all start using WhatsApp et al, but if 8/9 of our group message is on iMessage, why would we?