Tons of people have recognized my flash shirt and it is alot more than flash Gordon,Hell the movie flash Gordon was 1980 i don't who you are around that is recognizing in Flash Gordon today.Flash is one of more popular Halloween costumes and gets a couple of pop culture reference as the embodiment of speed.
A. He has easier suit to use than Aquaman,Wonder Woman,Superman
They got it right on 90's tv show,It will be easy to get right in a movie,Flash costume is right up there with spiderman that only get tweaks over the years, one of the best suits if not the best suit in comics
B.He has clean easy origin,He has solid rogue gallery.Flash is not a minefield like Green Lantern,Aquaman or Wonder Woman.No outer space blue aliens,No Atlantis,No Paradise island.Flash movie is way easier to do those movies
C.Speed is one of practical effects that Hollywood has mastered,see Heroes or No Ordinary family.
Flash is the safest bet for successful DC movie,It is least likely movie for them to mess up.Yes i will admit that flash does not have popularity of say Wonder Woman or Superman but that does not matter ,Was Blade popular before his movie? No and Flash has way more of a fan base than blade does, it is fair easier to screw up other characters than flash movie.