While Tarzan was a popular Disney film at the time of Kingdom Hearts 2's development, Disney is very selective about which franchises they allow to be used in other media. It's possible that Disney felt Tarzan's story and world wouldn't fit well into the Kingdom Hearts universe, or perhaps they simply wanted to prioritize other franchises.
It's worth noting that Tarzan *was* considered for inclusion in the game, but ultimately it was decided against. This decision was likely based on a combination of factors, including creative direction, budget constraints, and Disney's licensing agreements.