GSP is one of the greats simply because of his accomplishments inside the octagon; you cannot dispute cold facts and achievements made in the best mixed martial arts organization in the world. His top ten wins, dominance, longevity and title reign in general will make sure the history books remember him, well into the future.
However, nothing is more decisive than a finish, literally. You ether physically stop a person from competing by (T)KO or you make said person give up, which is basically pleading. It also removes other people, like the judges, from having an influence on what is going on inside the cage. The fact you have to go back to 2009 for his last finish means, although great, that GSP has been unable to put his opponents away, despite the majority of them receiving a long drawn-out beating as a result. And even if GSP, theoretically, could put them away if he wanted to, which I doubt, it means he can’t free himself enough to put himself into a position he can finish from without being extremely vulnerable himself.
This is also why he will never be considered the consensus #1 p4p fighter-nor be one of my favorite fighters. Not only for his lack of finishes, but the lack of risk taking, too. Even though these risks could elevate him to new heights e.g. accepting the Silva fight.