Word. Well outside of the trendy aspect, I think it's popular because it's easily accessible (available at their own stores and several dept stores), and the makeup artists usually (although there are exceptions) look on point. Sometimes when I go to Sephora I'm like, "Uh, honey, I know you aren't trying to sell me some makeup." They also do a lot of celeb collaborations and their limited collections make it more desirable because people know they won't be able to get it again, or they'll have to wait until they repromo it.
Also, their online chat help is actually helpful and the people responding actually know about the colors. Last time I did an online chat to find a new foundation, and she suggested I get NC40 rather than go to NW b/c NC has less rosy undertones, and I told her that I have yellow undertones. Then, rather than pressure me to buy it online, she said it would be best if I go to the store and double check, and lo and behold, she was right about my color.
I also like that their VivaGlam purchases go to their AIDS fund, and you can trade in empty containers/tubes for a free eyeshadow, lipstick, or lipgloss.