My bet is that this happened because they do develop both the Enlightenment desktop and the E libraries, which is a tremendous amount of work. Add to that that if they are a small team, they're not going to go relatively fast (afaik E17 took years...). Maybe it was the reason GNOME/GTK(+) and KDE (which began with an already developed GUI library) caught up.

But as I always say in this kind of posts, both Enlightenment and E are amazing and I so wish they were more rich featured and popular and, if I were the XFCE mouse and got fed up with the bullshit of the GNOME-ization/libadwaita-zion of GTK, I'd consider porting all my shit to E - it would be awesome if those two merged into one. GTK and E are both written in C, XFCE has a robust set of apps and a seemingly bigger team behind it...