He spoke about this recently too, the creators didn't like him/his directing but needed some reshoots on the pilot, so they asked his agent, assuming he'd say no as there was some bad blood (DGA rules mean you can't reshoot a directors work without their permission), but he just said yes and came back, much to their chagrin.

Also Tara Butters is married to Marc Guggenheim, and Kevin suspects that may be the reason he was never asked to direct an episode of Arrow (even though he loves the character and wrote comics for it), but did get to direct 3 episodes of Flash and 4 episodes of Supergirl.