I read somewhere that Black Widow might have been delayed because Scarlett Johansson’s contract entitles her to a percentage of the box office revenue. If the movie went straight to Disney+, she wouldn’t get her cut therefore Disney legally can’t release the movie directly to Disney+ without Johansson willingly giving up millions of dollars (which, let’s be honest, no one would ever do). I suppose Disney could have tried to renegotiate for a cut of the $29.99 Disney+ premium access price, but that would definitely be less than what a Marvel movie would normally pull in, again leaving Johansson very little reason to do this.