Porque ela abandonou o acordo e Ross sabe disso, como explicado em Capitão América: Guerra Civil :
Tony Stark: You let them go, Nat.
Natasha Romanoff: We played this wrong.
Tony Stark: 'We'? Boy, it must be hard to shake the whole double agent thing, huh? It sticks in the DNA.
Natasha Romanoff: Are you incapable of letting go of your ego for one goddamn second?
Tony Stark: T'Challa told Ross what you did, so… they're coming for you.
Natasha Romanoff: I'm not the one that needs to watch their back. [She walks away.] - source