Black Widow became the team leader, a role she took very seriously. Natasha always cared deeply about the well-being of those close to her, making the apparent death of Daredevil and then the likely death of her mentor Ivan Petrovich at the hands of Red Skull very difficult on her. Adding her teammates only made things harder.
https://www.marvel.com/characters/black-widow-natasha-romanova/in-comics

