Why didn't DC Comics choose the TV "The Flash" actor Grant Gustin for the role of The Flash in the Justice Leagues movies?
From Zack Snyder's own words:
“I just don’t think it was a good fit,” Snyder told the New York Daily News. “I’m very strict with this universe and I just don’t see a version where…that (tone is) not our world.”
“Even if Grant Gustin is my favorite guy in the world and he’s very good, we made a commitment to the multi-verse (idea), so it’s just not a thing that’s possible,” Snyder said. - thewrap.com
One more thing to be noted here is that keeping both Flash same also means they are making Arrowverse canon to DC Extended Universe. Arrowverse makes 5 shows per year now ( Arrow, Flash, Legend of Tomorrow, Supergirl and Vixen) and DCEU 2 films per year. Making them canon creates a lot of inter-dependencies.
MCU does keep TV and movie verse same but they don't have any major character crossovers. Their TV haven't crossed over to the movies yet. Flash is a main cast of The Flash show and keeping him in DCEU will necessitate a lot of changes on both sides.
Bustle.com also provided a good Aargument:
It might come down to it simply not being feasible for Gustin to star in 24-odd episodes of television and lead a huge solo Flash film at the same time. It would, undoubtedly, be a scheduling nightmare for all involved. But based on Snyder's comments, the truth also seems to be that he simply wasn't the studio's top choice for the role. Snyder clearly wants to create the DC Cinematic Universe from scratch, despite the fact that casting Gustin would have been an easy way to expand the DC Cinematic Universe beyond Man of Steel. And though it seems like Gustin would be an obvious choice for the role, it's important to remember that The Flash had barely premiered when the news of Miller's casting was announced. In other words, The Flash simply wasn't the hit it is now.