mostly because for many people it has the most comfortable defaults. Unless you go for the non-free ISOs Debian is too barebones and stable ships extremely old software, rolling releases like Arch may break at any point, Suse often lacks 3rd party support for proprietary software and so on.

Fedora is pretty solid but also very much on the experimental side, like shipping Wayland as a default versions ago when half of the applications didn't work right.

I've defaulted to Ubuntu because it 'just works', every other distro I just have to spend time to do what Ubuntu does anyway.