The character is expected to debut in the fourth episode of Season 3, and Barry and his gang of do-gooders will likely just be coming off of all the changes caused by him going back in time and saving his mother, and then going back in time again and reversing that action to try putting things back to normal in the present day.
OK, I think I’m getting the motivation on this now.
With his mirror gun, the introduction of the Mirror Master (played by Aquarius star Grey Damon), might be too much for Barry and crew to deal with right off the bat.
They’ll already be dealing with a slightly new world and a host of other bad guys, having someone with that much power right off the bat might make things a tad too tricky for the S.T.A.R. Labs team.