The main reason is because “Ubuntu” is popular. Ubuntu is so popular, that people that don’t even know what Linux is, when listen to someone talking about “linux” thinks that Linux = Ubuntu.
I don’t blame them for such ignorance.
People usually don’t like distro that are not Debian or Ubuntu based. They find it very hardcore.
There are a lot of tutorials, guides, articles, documentation for Ubuntu and these possibly will work on Linux Mint, Peppermint, elementaryOS… because they’re spins of Ubuntu/Debian. I don’t even mention about other Ubuntu flavors that are not the main Ubuntu with Gnome.
I have seeN a lot of comments like “Arch is for users that like to suffer”, “Arch users like to have breakages”, “Arch users are such nerds”. And we’re not even exactly “Arch users”. Manjaro is just like Ubuntu is for Debian. Manjaro is the Ubuntu from Arch Linux.
~> People tend to use Ubuntu (not only for it be a popular distro) but because it has a “Company” behind. I mean Canonical.
~> Some people prefer fixed releases over rolling releases (what Arch/Manjaro is) due to have some “stability”.
~> People like to use PPA’s.
~> People are used to Ubuntu/Debian commands, they don’t want to learn other comands.
~> People think that only Ubuntu is user friendly and other distros aren’t as user friendly as Manjaro. They’re afraid of changing distro and doesn’t like the way it works.