Imagine if Star Wars got pushed back to May 1978, or if Batman got pushed back to June 1990, or if Terminator 2 got pushed back to July 1992.
Once you set the date for a film and declare that date to the marketplace, a set of forces are set in action. You set that whole motion in action, doubly so when you have a Super Bowl trailer with said dates. The exhibitors, the distributors, the merchandising companies, the media, all the people involved, build towards that date. It's very perilous to change that date. I'm not talking a day or two, but once you start changing it a week, a month, a year later, three things happen. 1. People think the film is no good. 2. it costs enormous amounts of more money 3. you get out of sync with all the forces you that you used to market the film, to bring it to that moment where the expectation of the audience is fully realized by the movie. Once a studio picks a date, it creates a force that can work for you, or against you.
In WB's case, it always seems to work against them. No idea why studios continue to do this. It shows a lack of faith in your film. Sure, these studios can blame the pandemic, but c'mon, they've been doing this for the past 8 years.
What gets me is, DC/WB is always the first one with the idea. Batman v Superman (in production before Captain America: Civil War) and The Flash (in production before Spider-Man: No Way Home) could have put a stamp on certain film firsts, such as multiverses and multiple actors coming back to flesh out their interpretations of a character. Each time, Marvel beats them to the punch and it becomes old hat.
They had years to get a Michael Keaton Batman film up off the ground and have been promoting the idea well before a trailer even popped up. Now by the time the movie hits, people will have had enough of it. Then with him appearing in Batgirl next? Too much, too late.