The biggest share of Unity is 2D mobile games, something Unreal is not particularly suited for and I very much doubted that segment of the market will switch to Unreal.
All the Candy Crushes as well as several other games use variants of a custom internal engine that's managed by a fairly sizable central team.
It seems that Candy Crush predates the engine by a few years and that the engine was originally created as a hobby project by one of the King employees, although it’s seen commercial use on mobile by a number of studios and has received backing from King and others over the years. King are not currently listed as a backer of the Defold Foundation, but one of the board members is the “director of engines” at King.