X (not formerly twitter) is decades old and is built around deprecated ways of doing things as well as a lot of legacy functions.
Wayland is a relatively new project with the aim of replacing X as a more "modern" display server.
Wayland had some stability issues, but they've since improved.
I'm sure Wayland is good and all, but I can't be arsed replacing X yet. I don't really have any skin in the game, I just don't replace functioning components just because they're old (FYI, bash turns 35 this year). While X does what I need it to do, I'll keep using it. I'll probably move over when my distro does.
I'll leave the technical explanation to someone else.