>>122488036>>122484033I think it's not just about popularity, but also about long term influence and rounded representation.
Due to the order in which the characters were created, the Marvel Universe was built on four monumental franchises from which literally everything else is either derived from or centered on. The X-Men, the Avengers, the Fantastic Four, and the Marvel Knights (Spider-Man, Hulk, Daredevil, Doctor Strange, etc.). As such, their best developed characters and best stories are well spread out between these four "pillars". Not equally, but enough to consider the all when building a trinity.
Spider-Man represents the Marvel Knights and the best of Marvel in the Silver Age.
Wolverine represents the X-Men and the best of Marvel in the Bronze Age.
Captain America represents the Avengers and the best of Marvel in the Golden Age.
