Barclaycard used to issue US Airways cards, and those became American AAdvantage cards with the merger. But Citibank had the exclusive right to issue new AAdvantage cards. So once the US Airways Dividend Miles program ended in March 2015, existing cardholders got to keep their cards but no one else could sign up.
Today is the first day that new Barclaycard American Airlines AAdvantage cards are available.