XMPP never got its story together for mobile clients. Sure there are some configurations of XEPs that will eventually allow you a reasonable configuration that supports multi device, history, media, etc, but for whatever reason, gluing those pieces together into something sane has evaded our community.
But in fact most servers are mobile friendly, and have been for many years. You can dig into the data at https://compliance.conversations.im/ (Conversations being the leading mobile XMPP client).
In particular it never got its story together for iOS. ... and macOS more recently. It is quite good, on say, Android (Conversations). Rumour has it that XMPP is a lot more popular in parts of the world where Apple stuff is less popular. Those tens of thousands of XMPP servers are probably in those parts of the world.
There is an active project working to improve things on the Apple side:
Nothing on Windows had voice and video for a long time but Google Talk. More and more people used Google Talk and Facebook. Facebook never federated. Good mobile support took a long time. Google defederated. Facebook and Google cut off other clients.
And encryption, which is an addon and not the default.
Also, XMPP is a good example of a 'victim' of embrace, extend, extinguish. I remember Facebook messenger, hyves chat, Google talk and briefly even WhatsApp, all in my desktop chat app. They all 'did' XMPP, but then removed federation, features or killed it entirely.
