First there was the Ezra Miller issue. And it wasn't just because the Ezra-boycott people. WB's entire promotional campaign around the movie was basically designed to minimize Ezra Miller. WB downplayed them in the trailers. Ezra did zero promotions for the movie. Instead WB leaned heavily on Keaton Batman. But that doesn't really send a very clear message about what your movie is about.
That doesn't combine well with the other factor in that WB had basically declared the movie a vestigial limb to their new Gunn-lead DCEU. Sure, they paid lip service to the film but literally all other discussion about the future of the DCEU basically implied that the Flash didn't matter. But even this wouldn't have been the end of the world if people thought the movie was going to be good but that leads us to problem 3.
The general audience has really low expectations for DC movies right now. The DCEU movies have largely been critically underwhelming and disappointing to audiences. Yes, there are a few bright spots in there but they are definitely in the minority and they were mostly years ago. People just don't trust that these movies will be particularly good.
So you have a movie with a problematic lead, muddied promotions, that probably won't matter and likely isn't good. Not exactly a box-office hit formula.