10 Reasons The Flash Failed
1. No Zack Snyder
The source of The Flash's many productions-related travails can be traced back to the second film in the DCEU, Batman V Superman.
Even though Snyder appeared to be on the right track with BvS outgrossing Man of Steel and making nearly 900 million at the box office, BvS' reception instigated a period of reactive business decision from WB, effectively derailing their extended universe plans.
After Snyder's departure, WB threw out the multi-film arc Snyder had set the groundwork for. This arc would have included a solo Flash film, and while there is no way to tell how successful it would have been, being part of a coherent multi-film narrative would've likely made it more intriguing for general audiences.
With WB's game plan in constant flux, The Flash never had a chance to realize its potential.
The Flash has concluded the DCEU, and the Gunnverse will now take DC films into the future. If WB wishes to avoid another Flash-level bomb, they need to allow Gunn to fulfil his DC vision, and avoid knee-jerk decisions made to appease shareholders over film fans.