You are forgetting the biggest feature: You can actually leave an iMessage group chat. An SMS group chat will keep texting you as long as someone keeps it alive. To stop getting them you have to individually block every user.

LOL sounds like a good reason to be contrarian to me.

Statistically speaking, teenagers don't want to be contrarian in that way.

Outliers exist, but in the average teen doesn't want to be perceived as too poor to own an iPhone.

That was the missing bit of info for me. It's not just that the color is different, it's that a non-iMessage user in a group chat degrades the functionality for everyone else.