Sure, Cap has certain characteristics that makes him Cap. Leadership, all that.

But he's also an absolute badass who can kick the shit out of an elevator full of highly trained special ops types, all by himself.

Bucky can do that.

Sam can't.

Cap is the peak of what a man can be. He's fully realized, unlocked human potential.

Sam?

His wings are cool. He can do cool shit.
But his skillset is still a sidekick skill set.