Because BR made people feel. Widows may be technically better acted and have bigger names but BR strike the chord. Plus, actors love to nominate the small guys, unassuming actors who have no shot in individual categories. That was a big thing with Argo. All those Clea DuValle's who never have a chance with roles that put you on the map, the background players, were voting for those background players to get recognition.
BP wins this, though.