Comment by eyko

To be fair a slightly better comparison would be MMS, but even then it doesn't compare. It's simply because data is cheap, the app is free, and the rich media features that came before iMessage/RCS existed (voice, photos/video, attachments, group chats, voip/video calling, etc), and because it's multiplatform (ios to android usage is near 50/50 or 60/40 depending on the country)