Because the AUR is a pretty low quality repo. Not sure if anything has changed since 2 years ago, but last I used arch, the AUR was full of broken, abandoned, and unbuildable packages. The Debian repos, fedora+rpmfusion, etc, provide a comparable number of software packages with substantially higher quality, hence no need for the AUR. Fedora actually has COPRs which suffer from the same quality issues as the AUR for similar reasons.
Thing is, the AUR isn’t really meant to be your primary repo. You can really get anything into the AUR.
The reason why I love it so much is because if I need a package that’s not in the main arch repo (which tbh isn’t many), then I don’t need to bother going to some github page and compiling from source, I can just find it in the AUR and it’s all done for me. I did this with things like goverlay and it’s one thing that I immediately miss when I distro hop away from something arch-based.
I use Arch because pacman sounds cooler than apt, wakka wakka.
What makes you think there aren’t equivalents out there?
Gentoo’s Guru repository, for one, and any of the multitude of ebuild repositories available through the eselect repository command.
The AUR is not particularly special.
NixOS has NUR, but it’s not necessary because they take everyone’s pull requests in the official repo. I’ve been maintaining the software I use myself on the official nixpkgs, so I don’t need to use the NUR.
The majority of other distros value package managers that allow for complex graph evaluation of dependencies, and the ability to roll back. This is granted with rpm and Deb, but not for pkgsource, which is a pretty lightweight format compared to those.
As for AUR, the major distros (Ubuntu, Debian, Fedora) support 3p repositories as well. The main concern is security. IIRC one of major complaints for AUR in the past was that it didn’t foresee a strongly secure distribution system.
