Maybe because SMS became very popular in the US, and iMessage originally was just an extension of SMS from a UX perspective, 2 years after iPhone launched. It added functionality and fixed issues with SMS (multi-part messages, images, etc). This led to the dislike of "green bubbles" because it meant you were forcing a degraded experience on the other person. That started a positive feedback loop for iMessage.

My understanding is SMS never got anywhere as popular in the EU.