For my thought at least is... What could a Black Widow (or Hawkeye for that matter) movie do that couldn't be done bigger and better with other characters? For all their gadgets, they are really just highly trained agents. They can bring skills that are lacking with the likes of other Avengers. Or they could work well on a regular series, like Agents of Shield having Mockingbird right now. They can even do a comic book, because it's one of dozens of products the company would put out every month. But a feature length movie that is going to take over a year and millions of dollars to produce, with so many other, bigger characters to play with? I just don't see it happening. Black Panther I think is the bottom rung of the power curve to be possible to get his own movie. He's also usually depicted as a highly trained person with high tech gear, but his being the king of Wakanda and all that brings with it.
Now, I could see Black Widow and Hawkeye getting something like the Netflix series that are in production, or maybe a spin off TV mini series like Agent Carter. But a full AAA movie release I don't see happening.