I just asked you what made Dragonlance unique. You refused to answer, and instead just said "it lacks orcs."
There's not a whole lot. Magic coming from the moon gods. Magic being affected by the phases of the moons. No orcs. Irda. There's little to it that's unique.
Anyone can make a setting that lacks orcs. I have, multiple times. My current setting lacks any and all goblinoids--in fact, really the only sapient races it has are the PHB races, minus dragonborn and plus bullywugs and kobolds, and a small number of monster races like fey, undead, and dragons. Does that make my world more unique than Krynn?
Anyone can make a setting with moon phase magic, lances made to kill dragons and dragon wars, too. So if "Anyone can make it" is what it takes to remove uniqueness, nothing in D&D is unique and we should just not make any more settings.
And I can tell you this: if I had a sudden need to add a new sentient race to my world to accomodate a player... I would.
What could create such a need? A player wanting to play something doesn't create a need to add in a race to the world.
Not at all. It might even be easier, depending on the type of horror I want to go for, the type of players, and what sort of things they're OK with. Because (as an example) tempting them to resort to a more primal nature is prime horror material, especially when only one of those three races is a predator. And I don't even need to resort to that when all the standard horror plots are still available to me and would work whether or not the PCs have fur.
Funny how you're adamant you can run Discworld in D&D, despite how vastly different that setting is from the typical D&D setting, but think it would be too hard to run a horror game with non-human PCs.
A few things. I didn't say non-human PCs. But a rabbit, a turtle and a cat walking into a bar would be more in line with Discworld than Ravenloft. I can absolutely run both settings very well with D&D, because setting doesn't matter to the system, but that doesn't mean that every race or class will fit every setting. You need to curate the options to run D&D and accomplish various goals well.
I feel that you're just being a contrarian for the heck of it.
I almost never do that, and then only to people that have proven themselves to be trolls, which you haven't. At some point when dealing with trolls, I troll them back which sends them into a tizzy since they can't take what they dish out.
Not according to some conspiracy theorists.

Touche. I don't put much stock in what conspiracy theorists say, though.
But orcs aren't Martians and Athas isn't Earth. Why would orcs be any different than any of the other big, buff, violent races that already exist on Athas?
It kinda is, though. Throwing orcs into Athas is going to affect the entire history of the world from the point that they were eradicated to the current time of the Sorcerer Kings. Everything changes because of how interactions work. They would have a big an influence on Athas as martians living here from 10k years ago would have on Earth.
You keep making this claim. Prove it.
I have. It will feel different to me and the others(some of whom are in this thread) who it would feel different to. Done and done. I've proven that it will affect the feeling of the setting for some people.