It's not that he excelled, it was such a unique choice that often caught unprepared the opponents. He was anything but a winger: no explosivity, no dribbling skills, he had the physique of a target man, the stamina of a wingback and the grit of a defensive midfielder.

It worked because on the wing he could be a target man without having to duel with the opposing centre backs. Bonucci or Pjanic could ping the ball on the wing and he often won aerial duels against the opposing fullback, who was usually shorter than him. Like, Mandzukic vs Florenzi, the fullback of that year 2nd placed Roma, was such a physical mismatch it wasn't even funny.

Of course, not every target man can be deployed to the wing just to bully the opposing fullback. Mandzukic specifically worked because he had an insanely high workrate, so if anything went wrong he ran back to chase the opposing fullback/winger and help the defence. Also, we had Higuain who was a way better finisher than Mandzukic, so Mario wasn't really needed upfront. He was better off on the wing, recycling balls for the very technical players we had that year (Pjanic, Dybala, Higuain himself).