So they strip-mined it for what they liked and then discontinued the product line? So how then was the product line bigger than the Dark Sun product line? Remember you said one was "big sure" but the other was "huge."
If Planescape is baked into the current Forgotten Realms setting of D&D, how then is it any way likely that they would publish a second campaign setting focusing on the material they already made available - which is the argument this discussion about size and import of the two settings is based around supporting.
Planescape material had a weird dualism to it. On one hand you had planar material, expanding on the various outer, inner, and transitive planes. A lot of that was based on the old Manual of the Planes, and a lot of what was established in Planescape was brought forward to 3e and 5e. But you also had the setting of Sigil (and I think you can include the gatetowns in this), with its factions, weird geometry, planar crossroads, and Dickensian London feel.
I think the thing most people care about as specifically Planescape is the Sigil-centric material, which has been mostly ignored in later editions. Sigil has been mentioned, but mostly in passing. The only mentions in 5e are about a paragraph in the PHB, three in the DMG, and a mention in the description of a Marut in Mordenkainen's Tome of Foes.
As for comparisons between Dark Sun and Planescape, remember that Planescape was mostly regular AD&D. You had some new races in first the Planescape boxed set and later the Planewalker's Handbook, and each of the factions was represented as a kit providing some benefits and penalties, but a Planescape cleric was pretty much the same as a regular cleric (except they had to concern themselves with how many planes were between them and their deity). But Dark Sun pretty much rewrote the whole races and classes chapters of the PHB. The 2e PHB had about 25 pages about races and classes, while the Dark Sun boxed set had about 40 (though some of that was the rules for character trees, but still). In a very real sense, Dark Sun was a different game that ran on the AD&D engine.
The point of this is that if you want to play Planescape in 3e or 5e, that's pretty easy. You might want to figure out some way to represent the factions, but the rest is just lore. But if you want an authentic Dark Sun experience, you need to figure out at least how to handle races, defiling, elemental priests and druids, templars, psionics, and bardic assassins. Ideally, you'd also deal with crappy materials in some way, unless you go the way athas.org did in 3e and Wizards themselves did in 4e, and say "Sure, metal is rare, but Athasians are resourceful and have learned to make weapons and armor out of other materials that are just as good" (because doing otherwise would seriously mess with class balance). But that's why you'll find more Dark Sun conversion material than Planescape conversion materials for later editions – Planescape doesn't need it, but Dark Sun does.
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