Because...
^ This.
M'rk son of Mogh: again, the PTB are producing big screen DC films with a certain level of performer, and in no part of that mix are they going to put anything from Berlanti's low-rent crap into that--not actors, characterization, or anything else.
Your other statement--about the "type" of people they'd "rather cast"--is makes no sense as a would-be argument as it suggests WB knew about Miller's recent behavior/acts at the time he was signed, but that had not presented itself during that period, so your argument is rather fallacious. You get that, don't you?
..and i'm fairly sure in the event the DCEU's Flash is recast, WB/D has a wealth of talent to choose from, which is not coming from Berlantiland.