The main issue of GNUStep was that, when KDE appeared, the GNU project used GTK+ as a base for GNOME, instead of GNUStep. This basically killed the momentum behind GNUStep as a base for the official GNU Desktop. It is sad because GNUStep is incredible, the IDEs alone are more advanced than anything in GNOME-space, even today. Oh well...
There's a lot of "oh well" in the tech space. I was watching something on Pick OS yesterday, which I hadn't even heard of, which was a kind of filesystem/database/OS all together that was, if the comments on the video mean anything, quite loved.
But like BeOS (also loved) and other things just never got traction for various reasons.
I used Windowmaker for a lot of years as it was lightweight and I liked the UI, and always wondered "why not this? why ape Windows of all things?" The answer is usually something along the lines of "that's what people know," and I get it, but still.
I guess I miss the times when computers were cool.
The other main issue is that GNUstep is incredibly bug prone and fragile. Case in point, two years ago i installed GNUstep from source and then built a pixelart editor called PikoPixel. The app felt very clunky, like all GNUstep apps do (note that i'm using Window Maker as my main WM so GNUstep gets the best environment it can hope for). Then two years later with some OS updates in between it doesn't work anymore.
Meanwhile i'm still using the same mediaplayer binary i built ~7 years ago on a different computer against a C GUI library.