As someone who used to work with FLTK and loved the hell out of it: compared to QT, GUIs made in FLTK are visually ugly.
FLTK makes totally reasonable GUIs that folks can easily understand and that behave in sane ways. It "just" doesn't look "modern" and hasn't for decades... if it ever did.
I prefer how FLTK looks, but I guess the average joe doesn't?
In the earlier days, both Qt and gtk quickly jumped on the theme train, where you could customize the looks of widgets heavily, and IIRC FLTK never caught up there. I think the default L&F can compete with default gtk1/2 or Motif-ish Qt, but you couldn't go up from there or try to have a common look & feel between toolkits.
These days, that has gone one step further. Where a lot of people consider anything with "old-school" widgets to be ugly and outdated, and a more web-like cards/list-view based approach with highly customized and animated UI elements.
Qt with its QtQuick/QML parts seems quite competitive there, not sure how good Fltk 1.4 is here.
(Just repeating the way I see the "state of the art" here, I personally don't like that "proper" GUIs are now retrocomputing)
Both Gtk and Qt, but especially Gtk have those features. Qt also has a lot of corporate backing and jobs advertising. Gtk is the official GNU toolkit(?).
(I would also disagree on developer friendly or simple. But that's opinion of course.)