I'm not sure either. I think arch used to be one of the less popular distros (because of the more involved install process, solved now by the arch-based distros with friendly installers), despite having some of the best features, so it required more "evangelism", that's unecessary now. Arch-based distros are now some of the most popular ones, so its not necessary.

Others have commented on why its so great, but the AUR + Rolling releases + stability means that arch is one of the "stable end states". You might hop around a lot, but its one of the ones you end up landing on, and have no reason to change from.