The producers are probably still in negotiations with her. She's a bigger name now than she was when she played the role on stage and her agent probably also realizes the very thing you've pointed out about how there's no one better to do this. And I can't imagine the producers even gave a thought to doing this movie without her. So...they're probably going to have to pay her more than her current going rate for movies.
Erivo may also have other contractual demands to be negotiated that wouldn't have even been a thought several years ago.
It gains them nothing by announcing her attachment in the same press release. If they do it in two,with a bit a time between, they get two news cycles and two bursts of excitement, and gives the project the perception of momentum (either accurate or manipulated, either way it's good for getting the movie seen as a legit thing).
