How comes email managed to federate multiple incompatible systems (MCI,MS Mail,X.400,etc.) into an open RFC standard, where other protocols (instant messaging in particular) failed?
IM has a longer history than you might think. You could send instant messages to people on your network, long before there was anything commercial about it.
IM functions have existed on systems for a very long time. Some of them even had groups and various permissions associated with that. On top of that, there were overall user permissions with varied levels. They could send messages to other people in their same usergroups. An example would be /wallops or /globops, and similar commands. You still see that functionality built into things like IRC servers and some clients. There was also a /msg{user} command. There were also messages sent to the operator on duty, and things like that.
So, this doesn't detract from your overall point. I just figured you should know that IM wasn't created as a business, so much as it was a way to communicate with other people. It would be many years before people sought a profit in it, the monitorization of it was a bit later than that. The first monetization efforts, that I remember, were in selling products (and services) that made it easier on internal networks. For a while, Novell was pretty big in that space, but they weren't the first.
IM has quite a history. Someone should write a book on it.
I'm not sure of the absolute beginnings, but there has probably been IM since they first networked two computers together. It was probably the second thing they wrote and the first message was probably a fart joke.
You see some of that with e-mail and development around it as well (webmail for more features instead of clients and protocols, ...)
Email was quite fractured at one time but it did become pretty interoperable when it went mainstream.
(until Skype, Facebook Messenger and WhatsApp took over)
(Am I seeing a pattern here?)
The trend is that the people running the IM services want control of the clients, and so they shut off outside access or create new services that never support open protocols (see also: Facebook Chat, Google Hangouts, Apple iMessage).