For this example let's talk about Black Widow.
Most Marvel solo films cost between 125-200 million to produce. When you think about the fact that this is post-Endgame hype, a prequel, and a character that isn't one of the massive ones, its box office potential is probably between 600 mil to 1 billion (that's a stretch but it is the first marvel movie in ten months). Disney would need to recoup that cost. Putting it on Disney Plus would not result in that many new subscribers.
the alternative wouldn't be straight to streaming, it'd be straight to VOD.
If you want to see movies straight to VOD, you've got to be prepared to spend more money on them. You're not going to be renting them for $5.99 or buying them for $19.99 if there's no theater release.
But finally, thanks to the virus, film productions are being shut down and put on hold. The movies that are being delayed about most likely going to go in the place of something else that had its release date pushed back due to the production delay.