Too Long; Didn't Read
Pokémon Z would have been the third installment in generation 6 of the main series of games. Usually, with each generation, the Pokémon Company releases three games, with the third improving and adding new features to the previous two games. Instead, the franchise skipped the third game to start developing Sun and Moon (Generation 7) The game would have vastly improved on X and Y, which successfully transitioned the franchise into 3D on the Nintendo 3DS. Z was probably never released due to a lack of resources and, most importantly, time.