(humour)
Or you can just think that the Gods to whom the Wish spell is directed to are watching every battle as well as the development in whatever lands as "entertainment".
And they simply got bored after the 40-turn time limit and decided to will the combatants out of existence so as to relieve themselves of the boredom (additional "entertainment"). And to be impartial, they willed all sides into nothingness, so as not to favor anybody.
(/humour)
-Gateway103