I didn’t. All of the other VMs spent time training just like Mark did, except they did it for far longer, and the idea that his part human physiology gives him an edge is just a fan theory. We know for a fact that his father for instance spent centuries training so he could become a part of the world conquering committee meanwhile mark on the other hand is managing to beat a guy who’s part of a Eugenics program to make the strongest VM because he pumps iron from time to time.
Mark is way the fuck stronger than anyone else kind of like Goku or other Shonen manga protags because that is one of the types of series that influenced invincible and in those series The protagonists are often times way stronger than everyone else not for any really good reason.
It’s possible that he was one of many heirs and that there was no one specific person meant to inherit the Empire until selected. Another possibility is that he wasn’t a direct heir to him but rather two or three generations removed, meaning that Nolan’s parents would have very potentially not even realized his lineage. The last and, in my opinion, most likely explanation is that Robert Kirkman didn’t specifically plan out all of the minutiae of the series and just left this to the waist side or thought it up later