If you look at this without the Marvel fanboy colored glasses then the only reason why Black Widow didn't get a movie is because Fiege/Marvel/Disney pussied out at the thought of risking making a movie with a female lead. There is no other excuse.
Because while a Black Widow movie may not have been the best place to introduce another Infinity Gem, it could have very easily been used to flesh out the whole Hydra taking over SHIELD story that could have then led into Winter Soldier.
Man nobody was clamoring for a Black Widow movie, only reason why she gets brought up is cause no one wanted a Ant Man film (shyt, marvel was ho hum about a Black Panther movie until people brought out the pitchforks due to Ant Man getting the greenlight).