Don't underestimate the Ezra Miller factor. His actions turned plenty of people away too.
Not only that, but for most of the viewing public, they saw The Flash only really
one tone, in a God-awful Justice League movie where he was the 4th lead and annoying as hell.
To think all of a sudden, after 6 years, people not only would rush out to see a movie revolving around what was a bit character no one even really liked, but MULTIPLE versions of that annoying character was kinda assinine.
Spider-Man was perfect with the multiverse story because they brought 3 stars together, reprising roles that spanned three different generations. Spiderverse was amazing because they created a great character to center the story and then the multi-Spideys around him were awesome.
This movie had a Batman that was never beloved, another beloved Batman who was basically just used as a prop, they never shared the screen like the other examples… and Supergirl who never existed.
Just terrible decision making all the way around.