I mean it's kinda a chicken and egg problem but I'd argue that you've got it the wrong way around. I think the iPhone is so dominant only because of iMessage. It's a status symbol to have blue bubbles, and there are people seriously not dating Android users for example.
The iPhone got popular for different reasons but the only reason why it's so dominant in the states is that people started viewing blue bubbles as something desirable. And that is by design! Apple has deliberately made texting between iPhones and Android phones shit by not updating to newer official texting standards. They stuck with an SMS/MMS protocol from the 90s and early 00s, just so that people keep buying iPhones.