Not to analyze it too much, but when GC first started to be talked about, people were still suspicious of 20-weight oils, so the thickness of GC was seen as a good thing.
As gas prices have gone up, and 20-weights have proven themselves to be good oils, and more cars are spec'ing them, GC has become a very 'old looking' oil - a very thick 30-weight for cars that need a thick oil. So its fallen out of favour.
Still an excelent oil, though, no doubt about that! And yeah, as Dom says, in Canada, at least, there is still loads of it on the shelves....