It's not been explained just why Elidibus held Unukalhai (and Cylva, probably) in stasis until he needed them, but the most likely answer is that being a chessmaster he'd not be fond of having unnecessary pawns on the board. More actors than necessary increases the probability of errors; by keeping the number of players low he'd be better able to direct the flow of things and have less of a headache to deal with if things go awry.

Of course things do still go awry (the Flood of Light was not part of the game plan, and only got worked in thanks to quick thinking on Emet-Selch's part), but having fewer players on the stage makes things easier to direct.