There are a few reasons for this:
* Gameplay balance: The Black Gate is a massive fortress, and its inclusion could have drastically changed the balance of the game. It might have been too difficult to attack or defend, leading to frustrating gameplay experiences.
* Scope of the game: The developers likely focused on providing a diverse range of maps representing key locations from the books and films, but not necessarily every location.
* Gameplay focus: The Black Gate is more of a symbolic location than a place for battles. It represents the final confrontation in the books, and its inclusion might have felt out of place in the context of the game's overall objective.
While the Black Gate itself isn't a playable map, you can certainly find maps that feature the surrounding lands of Mordor, where the Black Gate resides.