He came in too late, he didn't fight top opposition outside of the UFC so he never had to adapt or evolve his game, and the UFC has far superior competition compared to anyone he ever faced at the time.
Yeah, Lima and Koreshkov are good wins but they were incredible young at the time. Also, GSP is objectively the most accomplished fighter in MMA history and fought mainly only the number 1 contenders of his era.
Have fun trying to sleep at night seeing Askrens rigamortus post Masvidal corpse in the Octagon.