There are two reasons that I see. One is that XMPP is an overcomplicated nightmare. The protocol was verbose, flabby, and hard to implement, and the server sof

There are two reasons that I see. One is that XMPP is an overcomplicated nightmare. The protocol was verbose, flabby, and hard to implement, and the server software was very hard to set up and run especially for novices. A person could not just install a server and start chatting, even if the server was just stand-alone let alone linked to anyone else. The second and IMHO more fundamental reason is that the Internet is a dark forest. Any open system that becomes sufficiently popular will be destroyed by abuse. If you offer a chance to make any amount of money whatsoever online millions of hustlers will rush into the void like gas molecules invading a cracked vacuum bell and will scramble all over each other to suck every last fraction of a penny out until nothing of value remains. So far only centralized managed systems have been (somewhat) successful at keeping the barbarians at bay. I'm not saying a decentralized system could never succeed here, but I don't think it's been done yet. (Cryptocurrency isn't an example as it's already been throughly destroyed for its original vision and use case by scammers. It's a great case study in exactly what I mean.) (Edit: the goal is not always money either. Read money as "value." Political propaganda, cult recruitment, weaponized disinformation, or just trolling for lulz all count as extracting value of some kind at the expense of the commons.) SMTP along with Usenet was one of the first casualties of this phenomenon, and I would argue that it did fail as an open system. You can run your own SMTP server but it's not for novices or people without time on their hands. You'll have to fight constantly to keep your IP out of blacklists and to keep spam away from your users. Spammers are in an arms race against huge companies with massive training data sets and entire teams dedicated to spam filtering, so this only gets harder over time. An independent SMTP server is easy prey. 99% or more of users use one of several large mail hosts. These are mostly Google, Microsoft, and Apple in the USA. E-mail hosting is cheap to free and the vast majority of people would rather someone else deal with the pain of defending them from spam.