A few (historical) contributing factors: FLTK doesn't follow "system" theming or look or integrate with anything. It's not the default of common "desktop eviron

A few (historical) contributing factors: FLTK doesn't follow "system" theming or look or integrate with anything. It's not the default of common "desktop evironments". It may also be less portable and lack bindings for the language the dev wants to use. 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.)