There was only one AT&T, and to connect a third-party phone to their network still required paying AT&T for access. There are dozens of digital messaging alternatives to iMessage, and Beeper doesn't appear to be interested in paying any of them.
That’s one interpretation. The other is that Apple doesn’t offer a way to pay for access. I would love to see legislation forcing the opening of protocols & force allowing compatible clients that aren’t abusive. I miss things like Pidgin/Trillium etc where you could just have the 1 app & not worry about what platform your friends were on. The current state of affairs is a major step back from the 90s/early 00s.