Given that along with Melissa Benoist, who is the definition of perfection as Supergirl, Grant is excellent as the Flash/Barry Allen, so why wouldn't he have been selected for the upcoming film?
Could it have been a scheduling conflict?
As someone who collected EVERY Flash comic and read them when they were released from 1972 through the late 1980s, I think I've got a good handle on the character, and its surprising that WB did not select Grant.
Three main reasons, and it sure isn't a scheduling conflict.
1) Your Givens Aren't Given
For many people, who are just as huge fans of these characters, these are not the best interpretations of these characters. For me, a huge Superman fan, I appreciate Benoist's Supergirl because she is, essentially, Superman with a skirt, doing Superman storylines as an adult, even taking on Superman villains as though they are hers. And while she does really good as that, that's not something that fits with Superman without one or both being redundant, but I'll come back to that.
Gustin's Flash has his moments, but he also has weaknesses, ones that I would not want to experience again if I didn't have to. He is not always the best actor, and the characterization of his character is often very mopey and unintelligent, and I don't blame the movie division for deciding that they did not what a mopey Flash with a history of making horrible decisions.
2) CW Characters mean CW Continuity
The CW DCU Continuity is extensive, as they've touched on pretty much everything other than Batman, Superman and Wonder Woman. As such, if you bring, say, Grant Gustin into the DCEU, that means that Amell is your Arrow. That means that Ben Affleck's Batman lives in a world where Damian Darkh shot off all the nukes in the world and they were removed. That means there are a hundred different ways to break continuity now.
That also means if you want to do Deathstroke in the DCEU, tha means that you have to bring back Manu Bennet and his origin is all about his obsession with Oliver Queen and his super solider serum made him crazy and see his dead girlfriend, and he's without it, lat time I checked. The DCEU can't do Teen Titans, since a few of the core Titans members are already in their mid twenties. Batman can't deal with Ra's Al Ghul, and on and on with a thousand decisions made by the CW crew, sometimes out of lack of budget or lack of time to tell a decent story, sometimes because they were made with no interest in making sure that DCEU had all options open to them, on top of having to justify extant continuity issues like the two Deadshots, and a Superman who is inspiring and hopeful and has been active for 12+ years with a moodier newer Superman all while explaining why a Supergirl who is just as able to help as Superman isn't on the Justice League along with Flash.
3) The scheduling actually is difficult on top of all that
Justice League filmed from April to October 2016, and while the first few months are the off season for 23 ep a year shows like Supergirl and Flash, they go back to filming in August, I think, which means you'd need to do almost half a season without a hint of your main star, maybe more than that if you think about the reshoots that are going on.
Making a whole universe requires planning, and CW's DCU is simply not well planned, despite how charming Melissa Benoist can be, or how earnest Grant Gustin can seem at times.
