I wouldn't say they don't want to modernise it. They have a theming engine that can use some other frameworks' GUIs to provide theming (like GTK+, Windows, and probably Cocoa) — but most distributions only have a hideously ancient version of GNUstep that lacks the theme engine.

I'm with you on the general idea that the GNUstep developers don't seem to be in a rush to take GNUstep out of the shadows and let it be a glory on Linux. More aggressively imitating Cocoa, adopting Swift and filling in some of its holes on Linux, and abandoning the wholly useless enterprise of source-level compatibility with the OPENSTEP APIs of the 1990s… these would be good.

And distros not having the old versions, too!