I especially like it because it's trying to follow UNIX philosophy. All of its tools are separate and you can use them outside of Xfce or replace them altogether. For example KDE is still not at the point that you can launch the panel outside of it.
gnome and kde people more likely to want to show off all the bullshit effects they're wasting all that ram on.
It's not that much lighter than them though. KDE has gotten really close, and truly light DEs like LXDE and LXQT destroy Xfce on that front.