I think it was the fact that he wasn't tied to his original style. So many fighters, even if they train other things, really only specialize in one thing. GSP saw his own weaknesses and worked at them. He never stopped learning new things. How many fighters really learn entirely new skill sets when they're already on top of the game? Not many.
What did Askren ever learn during his entire career? All he did was train wrestling. GSP trained everything, even gymnastics and other non-fighting physical activities just to train his body more.