And it was based or at least inspired on the previous XForms[1] toolkit – the one with way too many bevel shapes for buttons etc., not the XML thing. Probably at least partially done because XForms wasn't open source, and to provide a more "modern" C++ API.
Meanwhile, XForms has been open-sourced, but FLTK being in a different language and having evolved a bit since its creation didn't suffer from the problems Lesstif had and is standing on its own rather well. And despite being C++, it pops up rather often when you're looking for GUIs with decent language bindings (e.g. for Lua or Rust). Probably because it's less a moving target than Qt or, heck, Gtk.