For one, Gtk+ used to be advertised as a multi platform toolkit, with a port of GDK to Win32. Making Gtk+ widgets look like Win32 made sense for that.
Secondly, there were two groups of people using Linux and X11 in that era: one group who were exiles from commercial Unix, and another, faster growing group coming from Windows. For the former group, it was reasonable to want widgets to look more like Motif or Athena. The latter group wanted a more modern look, which could depart from old school Unix workstations.