the short version was that it was an up-and-coming federated protocol, with people working on clients and stuff, and trying to attract users. then everyone got really excited when Google decided to start using XMPP in their Google Talk product, because it would mean instant adoption by a ton of people! except now everyone just used Google Talk as their client, because it was ahead of the existing XMPP clients in terms of usability/UX, and UX work on other clients kinda died. but over time, Google being Google, they got distracted and started neglecting Google Talk, failing to enable TLS support while the rest of the XMPP ecosystem started making it mandatory, essentially cutting off all Google Talk users from the rest of the XMPP network. so now you had a Google Talk network that everyone was using with a decent-ish client, and an XMPP network that a bunch of people were using with clients that sucked, and they couldn’t talk, and all the momentum in developing a strong stand-alone network was lost due to people letting Google control the whole thing
Over the years, open-source has kinda turned from “let’s build a public commons” into “let’s do free work for big corporations” and it’s… not a good change, to say the least
Extensible Messaging and Presence Protocol (XMPP) is a communication protocol for message-oriented middleware based on XML. Learn more about it here.