There's a whole scene where all the Avengers try and fail, though Cap nudges it slightly because he's kind of worthy
Head canon: he purposefully didn't pick it up to be nice.
Yeah I like that explanation more because the alternative is like I said - he's "sort of" worthy when it really should be one or the other.
I heard an explanation that I liked, he is worthy but doing it for the sake of a party game prevented him from actually picking it up or something along those lines, but I gotta say the not picking it up to be nice is much better.
I prefer the theory that he was burdened at the time with the guilt of not telling Tony about Bucky's part in Howard and Maria's deaths.