Marvel only makes 2 or 3 movies a year at most. Do you think they would make more money from a Black Widow movie, or from an Xmen movie? There is no way it is more practical, or financially beneficial to do a Black Widow movie over another new project. Antman and Guardians of the galaxy didnt need anyone to introduce them, and neither will Captain Marvel. It makes far more sense to have an Avenger cameo in a newcomers movie, ( a la RDJ in Spiderman) than make a solo movie for C characters like Black Widow, Hawkeye, or Falcon.