Well, There are lots of reasons we're where we're at now.
That said, it's been pretty clear for a while Flashpoint was gonna be more like what Captain America Civil War was , i.e. basically an Avengers film.
Yeah, Cap was the focus to an extent , but really it was an Avenger's film which served to introduce Spiderman, Black Panther, and to bring several of the Avengers characters together.
WB abandoned the idea of a straight up solo , Barry Allen centered, Flash film, a while ago .
Whether they should have done that or not is another issue, but fandom has known that's been off the table for a while now.
And in this case, let's be honest, fandom really hasn't shown a great love or affection for Miller's Flash, and the GA really isn't banging down the door for a Miller solo Flash film. It wasn't as if his version of the character generated interest in his future exploits the way Gal's WW did from BvS.
Heck, there were alot of fans who were willing to show him the door not too long ago, so we can't act as if a solo Miller Flash film was universally wanted.
Again, not saying how what WB is doing is the right or correct way, but I'm pointing it that there hasn't been a great ground swell of love for this Flash from Fandom in general, let alone the general audience.
So, I can see why WB may have come to the conclusion that this version of the Flash can't carry a film alone... at this point. I know that may make alot of fans mad, but I think that's what they're thinking.
Ultimately, I think Barry Allen is still gonna be the central character who sets all these things in motion, so to that extent , its his film. He's our guide into the multiverse. It's gonna deal with his Mother's death , presumably , and his attempt to change events. Beyond that, it's speculation.