I think a large part of the difficulty here is that mostly people already use multiple messaging services out of a group including things like vanilla SMS, iMessage, Facebook Messenger, Whatsapp, and Snapchat, and some might eve use all of them, so they're hesitant to add yet another one into the mix. I'd personally be thrilled to delete all of the messaging apps on my phone besides one if the one that remained allowed me to message with my full set of family and friends, but unfortunately that isn't likely to be feasible anytime soon.

remember like a decade back there was a instant messanger aggregator? it was called like, chatzilla or something, i forget. but in the logic of today's walled garden system of tech fiefdoms, would never happen without some regulatory requirement.