I've discussed this in other threads, but it's primarily due to there not just being Planes of Existence for every alignment, but also for every in-between alignment. Like, I'm fine with the Abyss and Nine Hells, but Hades, Gehenna, and Pandemonium are really unnecessary. The same applies to most of the Upper Planes as well. Also, it doesn't make sense that there are two separate Planes of War (Ysgard and Acheron), and believe that the setting would be better if they were just combined into one Plane of War (like Shavarath from Eberron did), and, in general, there's just too much grid-filling in the cosmology and not enough consideration of how to run adventures in the planes.