It’s just always been the most valuable card. Before the 2004 trophies you mentioned existed. Here is a breakdown from a previous discussion on why the illustrator has always been the most valuable card:
Also something like the 2004 pikas wouldn’t make sense as they are a variant of the actual original trophy cards, the 1997 pikas. The 1997 pikachu trophy cards are the best contenders to the illustrator of being the peak grail.