...for the very reason that the version of Peggy Carter virtually the entire world knows is British, and one would assume that you have to BE American to be Captain America. And yes I know in the comics she was in some takes American apparently, but that was YEARS ago and I haven't seen any take on her which is isn't a former British SOE Agent. So if anything she should be Captain Britain, not Captain America.
I say this with the greatest of love and respect to the awesomeness who is Haley Atwell and her character of Peggy Carter, and the context with Steve dying makes sense in theory but I just don't see the code name suiting her at all. Just a bug bear of mine since the character first appeared in Marvel Puzzle Quest and is now apparently returning in Exiles.