It was subtitle The First Avenger to tag into the Avengers movie reminding viewers it was the penultimate movie in Phase One of the great MCU plan. Which was a pretty bold plan the likes of which hadn’t happened in cinema before. There was never any real controversy in Marvel Studios about the name, they’ve got the sales figures they knew Captain America was a globally popular character.
It was the lead in to the avengers, but the studio offered it in other countries without the Captain America title, just the subtitle.