Because that's the established Planescape canon and that must never, NEVER be deviated from.
Well, it was, wasn't it? D&D4 deviated from it more or less completely, really. The Great Wheel hasn't been D&D for four years. So it sounds to me like your point is less that the developers or community see Planescape as inviolate and more that you like the Astral Sea more and are sorry to see it go.
Which I agree with, honestly, but you should avoid hyperbole. Particularly if your stated goal is to avoid edition warring.
Constructive versus destructive change.
Careful, #Shemeska, your bias is showing. Planescape did a lot to update the AD&D1 Manual of the Planes, but the fact is that D&D4 filled in a lot of gaps that even the Planescape canon left open, and the rewriting of the planes did more to unify D&D under a single cosmology than all of Planescape's ham-handed shoehorning of Athasian halflings into Sigil's Hive.
More isn't always better. It's true that the Astral Sea cosmology is simpler than the Great Wheel, but I'd argue that the change did more good than harm. Aside from a few truly numbskull decisions like combining the erinyes with the succubus, and thinking Chaotic Good is not a thing while Lawful Good is SUPER GOOD, the new cosmology has internal consistency that Planescape lacked precisely /because/ it was built on some very shaky foundations from AD&D1 and contradicted as few of them as possible.
I love Planescape, but it's clear to me that the designers of D&D4 looked back at the history of D&D cosmology and made some hard decisions about unifying concepts to give the game a home that was exclusively its own, rather than just reprinting what had come before. I really appreciate the difficulty of that effort and the quality of the result.
4e... recycled classic names for things with either only a passing resemblance to the earlier creature
Generally speaking, when someone starts saying a new thing is bad because it is not like the old thing, I start to get nervous. Do you have /specific/ complaints about the D&D4 identification of eladrin or archons? Because again, I find the new stuff more internally consistent than the old material.
There was continuity between the planes of 1e, 2e, and 3e as the same classic cosmology slowly evolved.
By the same token, I can argue that there was a lot of garbage given a pass because of historical provenance, which is never -- scratch that, /rarely/ -- a good reason to do anything.