I've been using ubuntu for more than 2 years now and would like to know why arch is so popular. What advantages does it have over other popular distros?

Pure arch is only a minimal installs,
so it becomes the system you want.
Ubuntu throws all kinds oft stuff at you, even if you just want to install,
lets say, xfce.

AUR

Some of things I like about arch.
Bleeding edge,
Aur,
Archwiki,
Minimal - I get to decide what I want to install, my installation is year old and it just has < 600 packages as of now.
Makepkg

Mainly pacman and aur.

And you can look down on those who don't use arch, unless they use gentoo

Because Minimalist installation process

Near-complete customization

Excellent package management

Offers bleeding-edge software; always up to date (hence, rolling-release)

The very complete Arch User Repository; every package you want is highly likely to be either in the official repositories or in the AUR

Godlike wiki