The problem is that WB as a studio can't put that much time/money/effort into one franchise. WB is more than just DC alone and a Marvel-style approach would force them to drop other projects, which they couldn't sustain given the existing contracts they have with multiple production companies. Marvel Studios' big advantage is that their only concern is Marvel movies and they can just pick whichever projects they feel feasible. Meanwhile DC has to convince the studio heads at WB that a Wonder Woman or Aquaman movie would make more money than say, a Pacific Rim sequel or a Will Smith blockbuster, and they'd need to do so for every movie, every year! Add that a movie like Man Of Steel was considered an 'under performer' because it didn't come close to the one billion expected from A-list superhero properties (Batman, Spider-Man, X-Men) and you can imagine that WB has little interest in pursuing a Flash solo movie with the risk it's going to end up gaining less than 500 million worldwide when they can develop three other movies that most likely will.