What's stopping it from being on movie screens by the Christmas?
Being safer than sorry. Betting on a better market next year than this year. And also who knows how extensive those reshoots are going to be. That’s what warners wants to do.
Also they pretty much said all their movies planned for this year are going to hbo max. They didn’t want Batman to be a part of that bunch and you can’t move the date up now and tell people “all movies except the Batman are going to hbo max this year.” This is their strategy for their releases this year and clearly they didn’t want Batman to be a part of that. And it makes sense. The Batman franchise and the Harry Potter franchise are their golden geese. They want to maximize profit.
Sony and marvel are going a different route with spiderman and that’s their choice.