The idea is that the Phoenix egg has to go through intense heat in order to hatch.
@Higs, that's not to say that Cremate decks will always run dominant. Cards will come out in the future that will let other decks be on par or even better than them. The best way to balance this mechanic, of course, is to release other cards that help preserve the balance the game.
While it may not be needed at this specific moment in time, I'm sure you can agree it is a fun idea and has possibilities in the future at least.