It’s Cannonical. They prefer implementing everything themselves fast, rather than developing a more sustainable project with the rest of the community over a longer timescale. When they do that, there will be very little buy-in from the wider community.

Others could technically implement another snap store for their own distro, but they’d have to build a lot of the backend that Cannonical didn’t release. It’s easier to use Flatpak or AppImage or whatever rather than hitch themselves onto Cannonicals’s homegrown solution that might get abandoned down the line like Mir or Ubuntu Touch.