Thor said that the reason the Guardians couldn’t use Stormbreaker, was that they were not gods like him:
QUILL: Wait, what kind of weapon are we talking about here?
THOR: The Thanos killing kind.
QUILL: Don't you think that we should all have a weapon like that?
THOR: No. You simply lack the strength to wield them. Your bodies will crumble as your minds collapse into the madness.
If that's true, how did Captain America hold it?