ok so what do we know about captain america, 1 he was a smalltown brooklyn boy who was chosen to undergo the super soldier program in which he was infused with a serum that came from the tesseract, because he was in quote " a good man". 2 as captain america whenever steve throw his shield besides it getting stuck in a doorway, most usually comes back to steve, and the only other people who can hold his shield are peggy carter, bucky barnes and the red skull, all 4 of which the shield has deemed worthy enough to pick up,
now here we see cap trying to actually lift hors hammer, when u watch the avengers age of ultron extended trailer u can slightly see cap being able to budge the hammer but why is that ill explain.
the tesseract may in fact be the space gem that we all have seen in the 2012 avengers movie, but we know that it came from asgard because thor and loki and their father all know how to use it, so my fan theory is that like thors hammer which can only be lifted by thor or someone who the hammers deems worthy enough. captain america is of some asgard bloodline now but when cap tried to actually lift it the hammer didn't recognize him to be fully asgardian