Asgard is a realm within the broader plane of Ysgard (as are Alfheim, Vanaheim, and Jotunheim, for that matter). Just because the Norse gods are a major presence on the plane, that doesn't mean it belongs to them alone.
Right. If I go to rando #345 on the street and ask them "who lives in Asgard" What do you think their response will be?
Sure, they renamed it Ysgard and it only contains Asgard, but it is the Norse Pantheon plane. That's what anyone is thinking when they first read that name. And they are right. It contains Asgard, Alfheim, Vanaheim, Jotunheim as well as the World Tree Yggdrasil (that is how it got the name Ysgard, which they claim is just Asgard in the Norse tongue. They combined Yggdrasil the world tree and Asgard, the home of the Aesir into one name)
It doesn't matter who else lives there, it is clearly the norse plane and contains 90% norse mythos by weight.
Heliopolis, the major realm associated with many of the Egyptian gods, is in Arcadia.
And look at this in response. The Egyptians don't get an entire realm of existence, they get part of Arcadia. And their plane is called Heliopolis.
Do you know much about Greek Myths and history? Like how "polis" was the greek name for city, and Helios was the Greek god of the Sun. Wanna guess what Heliopolis' other name is in DnD? Oh, and Arcadia is... a realm in Greek Myths.
So, the norse get an entire plane of existence, containing the vast majority of their myths. The Egyptians live in a greek plane of existence in a greek city, named after the Greek god of the Sun. Do you see why I think that is not great?
Also, there's no real reason divine realms within a broader Outer Plane can't be accessed directly from the Astral anyway. Passing through a color pool to Ysgard is going to drop you at a specific place within the plane anyway, so why can't that place be within Asgard?
There's no reason the Spelljammer/4e model of Divine Realms accessed directly from the Astral can't function just fine alongside Planescape's Divine Realms accessed through the various Outer Planes.
Definition, scope, and weight.
Sure, a portal in the astral could drop you in Asgard, that's not the problem. The problem is that Asgard is a full plane of existence with multiple sub-planes. Meanwhile the Egyptians are bumming off the Greeks secondary realm, and The Olympian Glades of Arborea contains both the elven realm of Arvandor and Mount Olympus, because obviously Zeus and Corellon are neighbors, right?
Well, the greeks also get the Grey Wastes of Hades (weird how an entire plane is named after a single god) and Elysium and Tartarus in Carceri (which is an italian word, tying it to the romans I bet)
So, the Greek myths have half a dozen planes of existence, the norse have a plane of existence. Now, who wants to make a guess as to the two most popular mythologies when DnD was being made? Even if the realms are shared, or can be found in different ways, they are still listed as entire fundamental planes of existence.
However, in the World Axis model, where all divine realms are just places in the Astral Sea, then they all have the same weight. None of them are more important than the other, because you can place all of the locations needed for the divine mythos of the pantheon in a single location (their divine realm) and those Divine Realms are all equal. It is a far cleaner and more balanced version.