It really just comes down to the differences in goals and philosophies between each distribution. Some distros have large curated repositories containing most of everything a normal user would want to use. That’s what people expect from those distros, and people use them because they want that experience. Likewise, people don’t use arch just because it has the AUR. They want a more DIY experience, and arch provides that, with the AUR being an essential part of how it works.
You’re not going to get arch users to switch to ubuntu or whatever by duct-taping an AUR clone onto it. Furthermore, I believe trying to make one distro “to rule them all” that attempts to appeal to every niche would be not only a train wreck technically, but an abomination, antithetical to the principles of the OSS community as well.