Unless something has recently changed, XMPP also failed on the server side for normal users:
- Finding a practical provider is nearly impossible, because all of the major ones closed their public XMPP service years ago, and most people don't know what qualities to look for in a small provider, let alone understand why those qualities matter.
- Even if they get help from a friend and manage to find a reliable server that supports the magic combination of XEPs that are needed to make things work, chances are that server is no longer accepting new users.
- Even if they manage to get an account on a featureful[1], trustworthy, stable, free[2] server, many don't last more than a few years, so there is a high chance that the user's new online identity will evaporate before long.
[1](Required features include end-to-end encrypted group chats with offline delivery, among other things.)
[2](Yes, free. That is critically important for mass adoption today.)
I'm not a normal user, in the sense that I have far more technical knowledge than most people who use messaging services. I actively used XMPP for at least a decade; probably closer to two. I tried last year to piece together an XMPP-based solution that my friends and family could use, but even I was unable to solve all the problems I found in a way that would work for normal users.
And then there's the client side, which is also problematic, as soupbowl said.
This has been the state of things for quite a few years. The only people I still see pushing for XMPP are people who have major blind spots. Some of them have experience only with small groups, and have not considered how a whole world of people conversing presents different challenges. Others (usually tech folks) have had a good XMPP provider and comfortable client software for so long that they have forgotten how much knowledge it takes to get set up in the first place. A few are people who run small for-profit XMPP services or develop XMPP software, aka conflicts of interest.
Dear Jabber, I'm sorry. I liked you. I had high hopes for you. I might even still consider you for niche purposes. But as a general purpose messaging network for regular people, you have failed. It would take a lot of effort and resources to make you viable, and I don't see any sign of that coming. The closest you came was when Google and Facebook embraced you, but those days are long gone.
Meanwhile, Matrix is succeeding. The rough spots are being fixed. The bloated clients are getting competition. The missing features are appearing. Even as a work in progress, it is already usable for some of us, and it's consistently moving in the right direction. Importantly, I can tell a regular person where to get it and (often with no help) they can be chatting with me in a few minutes.