I really do not see that, the Lady really does not like or care for peasants and she does not bless them, also if the smiths had magic they would have been snatched out of their cradles by fey.
I tend to find the argument that the Lady doesn't care about peasants uncompelling. If she didn't care about peasants, why have the knights protect them at all? Plus the birth-swords are going to knights.
No? They might be taken by servants of the Lady, but AFAIK, that's it for those with magic who get taken away (and canonically, not all of them go off with the Fay Enchantress, at least some go to the Empire and study at the Colleges).
It might be for rule reasons. Monsters can't join units, they can only be Ridden, and only by specific characters. Since Ariel isn't playable, there is no one to ride him, so he's just a solo unit (although he has a unique rule that lets his Hounds join him).
Never being part of another unit might be for balance reasons, since Orion's pretty damn powerful stat wise. It means he can never benefit from "Look Out Sir!" enhanced version (I think he can still benefit from the nerfed Look Out Sir!).
As Boney says, it's mostly about model size, and Orion has a big model. He's like twice as tall as any other Elf, and is larger than Drycha. Can't benefit from any version of Look Out, Sir! though, because the nerfed version still requires a unit of 5 rank and file models of the same troop type within 3".
It's interesting to see you say that you think Orion is powerful, because general consensus is that, in 8th, he's an overpriced glass cannon you should skip.
Ariel was playable back in 5th edition. Not since then, which I never quite got.
Ariel last got stats in 4th edition, and while the setting was recognizable, the game certainly played very differently. Ariel the giant moth might not have sold well enough to justify it either.