XMPP has been an option for decades, if your contacts aren’t using it by now, they arent going to. And with communications tools, both parties have to agree on a tool. Even if one party doesn’t care about privacy or security.

Raw brute force security isn’t the point most of the time, and ease of use and simplicity of setup are going to be major factors in adoption. Signal is much easier to get started with for most people than XMPP.

Its possible that being the platform meant they wouldn’t have been bought up.