Where do people use iMessage? Do they refuse to be friends with Android users? The whole phenomenon sounds bizarre, like people who only communicate with people using the same carrier or something.
It's random... if you have three friends who happen to all have iPhones, that thread will be iMessage only forever. If one of the participants has an Android, it will use MMS. I've been in friend circles where people were using mostly iPhones.
Same in my social circle - in fact if someone is on Android, we hate using text in general and go straight to FB messenger (or GroupMe depending on the size, but less so recently).