He got one action moment at his end, and then they try selling you on his dead being this huge thing. For that to work you kind of need to spend more time with the character, you don't split the time with another character (who fills the same role) when you're trying to build to something within the story with one of them.
It not that she steals his thunder, it's that she takes up time and comes off as useless. Whedon should have been able to see: oh, I'm trying to do this thing, maybe I let the audience hung out with this guy a bit more. How do I do that? Oh, this Hill character's stuff could be Coulsons. Why not Black Widow? Black Widow is one of the main characters in the movie, and that scene helps establish her and what happens later with her and the Hulk. Hill is just kind of there not doing anything.