Motif was the real "classic" API. But let's do a little justice to GTK1. Motif was still a proprietary library when GTK1 was released. GTK1 was also already easier to develop with, and the default look&feel was somehow better. For some reason, all the widgets in the Motif UI were huge. Given the small resolutions of that time, it was very space inefficient.
The free clone "LessTif" already existed when "GTK+" 1.x came along.
I agree that GTK was much easier to program, and had a better feel but the look was mimicking Motif except that every widget instead used a shaded button-style border — and that looked very ugly IMHO.
However, GTK 1.x supported theme engines. Back in the day, my theme for GTK 1.x was one of the most popular, giving programs a look reminiscent of NeXTStep and Windows 95.
The engine also (optionally) hacked the GTK widgets, adding triangular tabs and subtly improved menu behaviour. When GTK 2.0 came along, it had a much improved default look that felt like its own thing, and also the new menu behaviour so my theme wasn't needed any more.
Xaw was also really awful to program in. When I tried using it once I ended up reverting to just plain Xlib and was much happier, even having to build my own widgets out of low level primitives.