It has nothing to do with the protocol, it's the penetration of the services that used it. In the 90s email addresses along with domain names became part of pr

It has nothing to do with the protocol, it's the penetration of the services that used it. In the 90s email addresses along with domain names became part of professional fashion. Everyone wanted @mycompany.com addresses, and that created one of the obstacles for walled garden email. And ISPs often offered email-addresses to subscribers. "The Microsoft Network" was an attempt at a walled garden that failed. Hotmail etc had no choice but to support the existing open protocols. Hotmail was just one of many free webmails. No instant messaging network used "global identities" (or hierarchical identities/addresses like email). The early-mover ICQ was a walled garden, and it died quickly when it became outdated. ICQ logins/identities had no relevance to anything else. If instant messaging platforms that wished to form a network had existed, then adopting some protocol would have been trivial. Creating new a network of services/operators competing with already an established platform is not trivial.