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.
This isn't to say there are no good things on the Great Wheel - I mean for example domains of the afterlife are fine but why so many? And I like Mechanus and it's an excellent realm for RP - but it doesn't really feel like an
afterlife realm the way Nirvana and just about all the original Outer Planes are.
Then there's the symmetry of it - which reinforces what are IMO morally toxic notions of "the balance between good and evil" because the cosmology is deliberately and inherently balanced or is being deliberately balanced by some twisted overgod like Ao. Even the Realms broke away from the Great wheel.
Meanwhile the World Axis, by not presenting things as equals is a lot less box-ticky than the Great Wheel.
I also start looking at it and seeing that the two closest realms to The Mortal World are The Feywild/Faerie of urban fantasy and The Shadowfell/The Shadow Realm. That
immediately gives me far more pointers and inspiration for going extraplanar than the Great Wheel ever did; for all James Wyatt talked about not tripping through faerie rings in 4e the World Axis encourages it by making faerie a thing. And there are direct links indicated to The Mortal World. Then we have The Elemental Chaos. Things like the Elemental Plane of Air, being air as far as the eye can see are ultimately pretty sterile. The roiling Elemental Chaos allows you to do almost everything you can do with an elemental plane by having bubbles of that element - but is also much more able to support cities on the edge of multiple elements (rather than having para-elemental planes as the boundaries of two) or with swirling regions of them. It's harsh and dangerous but somewhere much better able to support life. And then we have the Astral Sea which supports afterlife regions without needing five separate versions of hell of which only one is The Nine Hells.
It's worth noting that the 5e Realms cosmology may call itself the Great Wheel but (like so much of 5e) it's probably more 4e than any other edition; the Feywild, the Shadowfell, and the Elemental Chaos, all invented for 4e are pretty central and the Wheel, replacing the Astral Sea, is in the distance.