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.