It could be that he did not earn the right until he cleared his conscious. Or it could be that in Ultron, he really could have lifted it, but he did not want to embarrass Thor. A friend of mine told me that there have been a few people who have been able to hold the hammer and Stormbreaker. That's why I think Thor said "I knew it" when Cap picked up the hammer. Either way, it was cool when Cap started using it.