Besides, 20th Century Fox’s Red Sparrow basically resembles what a Black Widow movie would look like if we were following along with Natasha from her KGB days, so Marvel might as well take steps to ensure that her standalone story doesn’t look like a duplicate.
When we last left of with Black Widow in Captain America: Civil War, she was on the run after helping Steve Rogers and Bucky Barnes escape to track down Helmut Zero.
That would be a much better path to go down rather than simply wind the clock back fully.
Aside from rocking blonde hair in Avengers: Infinity War, we have no idea what’s in store for her in the movie, though she evidently survives, because she’s rumored to be battling Japanese gangsters in Avengers 4.