You're asking specifically why would any particular iPhone user want iMessage to be exclusive? It's not that they actually affirmatively want it to be exclusive, it's that they simply don't care. They either only communicate with other iPhone users, or a few green bubbles don't bother them. Most of the advocates for iMessage on Android are Android users. It's not complicated. Why would Apple put in the effort to open up iMessage when their customers don't care about it and only their competitor's customers do?