In my case because the whole thing feels very artificial and looks like a box-ticking exercise where they've specifically added extra versions of heaven or hell just to say that they are there. What am I supposed to do with the LGG heaven of Bytopia/the Twin Paradises and how is it not redundant when put between the Seven Heavens and Elysium. Or the block between Acheron, the Nine Hells, Gehenna, Hades, and Tartarus. That honestly reads to me as if someone just looked up "Hell" in a thesaurus and took five different words.
Exactly this. When I first looked at the Great Wheel Cosmology in 5e, I thought "Oh, cool! Look at all of these planes of existence! I'm sure that in the near-50 years of D&D's history that these have all been well-thought-out and detailed to have them included in campaigns, especially as places for the PCs to journey to!"
Then I spent about an hour or two reading about the planes in the DMG. I quickly fell in love with a few of the planes (Mechanus, Limbo, the Feywild and Shadowfell, the Astral Plane, the Elemental Planes, the eternal war aspects of Ysgard and Acheron, and a few others), but also began to realize that a lot of the planes were lacking in description and/or largely redundant with other planes. I really liked the idea of Carceri as "the Escape Room to End All Escape Rooms", but it barely had any description in the DMG and other official sources, so I never really ended up using it (I used it one time, and it was because of an extremely unlucky roll from a PC against a casting of Prismatic Spray).
I also got a few questions, like "Why the heck are there 7 different flavors of Heaven and 7 others for Hell, but no Plane of Mirrors, Dream/Nightmare Realm, Death World, Twilight Forest, or . . . [etc]?", "What's the difference between the Beastlands and the Feywild?", "Why are the embodiments of pure chaos [Slaad] evil, multicolored frog-men and not Fey?!?!", "Why do we have two different planes of War? [Ysgard and Acheron]", "Why isn't there a plane of Dragons and another of Giants?", "Why isn't there a Mount Olympus Plane if there's a Hades, and why isn't Hades the Underworld!?!?", "Why do we need a plane for Neutral Evil fiends [the extremely boring and uninspiring Yugoloths], and why is the plane for Neutral Evil fiends not the Neutral Evil Plane of Existence [Hades]!?!?" and "Why the heck are all of these redundant planes of existence different from each other, and not just separate parts of different, bigger, and more interesting plane of existence?"
I like Planescape (not from experience, I like the idea of it, though). I like planes-hopping, extraworldly weirdness. However, there's just too much overlap in the Great Wheel, and it just felt like grid-filling, "Okay, we have one plane of existence for every one of the 9 alignments, now let's do another for every in-between of those different alignments!". There are times when a "grid-filling" game design mindset has been beneficial, but the Great Wheel isn't one of them.
/end rant