XMPP missed the boat largely because it couldn't handle multiple clients correctly for years - the default is to deliver messages to one of your clients, you need an extension to do the sensible thing, and that extension spent years in bikeshed limbo right as smartphones were taking off and people started wanting to use the same messenger on their phone and computer at the same time. (I've heard that performance/battery issues from XML validation didn't help either)
Personal speculation but I blame the "everything is an extension" model - it was meant to reduce fragmentation and allow clients with different featuresets to interoperate, but in practice adding a new XEP seems to have all the downsides of making a change to a non-extension-based standard (you still have to get all the clients to agree) and none of the upsides.
Personal speculation but I blame the "everything is an extension" model - it was meant to reduce fragmentation and allow clients with different featuresets to interoperate, but in practice adding a new XEP seems to have all the downsides of making a change to a non-extension-based standard (you still have to get all the clients to agree) and none of the upsides.