[BanazirGalbasi]

I think it's because Xubuntu was a lighter alternative to Ubuntu when it ran Unity desktop. I don't know how Gnome compares to Unity, but at the time Xubuntu was far better for low-end computers than vanilla Ubuntu so the reputation of XFCE was justified.

[johnisgood]

Xubuntu was lighter than Ubuntu (when it had Gnome), at least that is how I remember it.