It isn't difficult to grasp, it just is difficult to CARE.
Let's just compare for a second. The 4e model had seven major parts to their cosmology. The Prime, The Ethereal, The Astral Sea, The Elemental Chaos, The Abyss, The Feywild, The Shadowfell. There were 19 major Divine Realms in the Astral Sea, including the Nine Hells (as one) and 11 major Elemental holds. These were all defined as being the realm of a specific being. This realms belongs to this god, this hold belongs to this titan, and on down the line. This would mean there are at max, 37 locations, with the Infinite Abyss being a problem child.
2e/3e cosmology? Well, I could say that it has ONLY 25 major parts... but even that isn't fully accurate. You have the prime, the etheral, the astral, Then the Four elemental planes... except there are six elemental planes, because Positive and Negative, and then you have the four Para-elemental planes (Magma, Smoke, Ice, Ooze) and then eight Quasi-Elemental PLanes (Lightning, Radiance, Mineral, Steam, Vaccuum, Ash, Dust, Salt). Then you have the Outlands and sigil, and the 16 outer planes.... except those 16 planes are divided into 53! different sections.
So, where maxing the 4e model gives you 37 locations, maxing the 2e/3e version gives you 76. It is double. And it isn't like it is difficult to understand, because it is box filling. Six elemental planes, then the four borders between the main four planes, then the eight edges that touch the positive and negative. It is easy to understand the boxes. It is far more difficult to try and make Earth, Earth and Water, Earth and Fire, Earth and Positive, Earth and Negative actually interesting to adventure in. And then do the same for Air, and then do the same for Fire, and... Seriously, it is just excessive. Same with the outer planes, You have good, then good and a little law, the law and good, then law and a little good, then good and a little chaos then good and chaos, the chaos and a little good. And sure, I can draw the model and explain how each subdivision fills the checklist, but it doesn't make for good adventures.
It isn't the instinct to make a good location for storytelling, it is the instinct to categorize and break down in compulsive detail every possible mixture of things.