Standard D&D campaign setup: orcs are here, fiends are usually elsewhere, and only rarely able to make incursions. If your campaign has the fiends here, I submit that you may just be using them as substitute orcs.
I submit that you have your position backwards: that orcs have
classically held the place of demons.
As far as "standard D&D setup" goes that's a weak argument. Since I can't think of an official campaign in recent years (lets say, the last decade) where orcs were innately evil. As bad guys? As bad guys whose motivations we never analyzed? Sure. I don't think that's the same as saying they're born evil though.
De gustibus non est disputandum, but what I'm getting at is that the line you're drawing is kind of arbitrary, especially since you accept that other kinds of creature can be innately evil. Somebody else might just as easily draw the line elsewhere. Why are orcs on the "not innately evil" side of the line? Why are/aren't giants? Dragons? Undead? And so on.
Of course there's no accounting for taste. And certainly someone
could draw the line elsewhere. In none of my statements have I not prefaced my comments with "I think..." or "IMO..." or "My preference is..."
My orcs are not innately evil because I find it more interesting for them not to be. It makes more sense to me that a mortal race, requiring food, rest, companionship and all the other functions of the flesh would have rationalized motivations for seeking the things they need, just like anyone else. Immortal races that do not require food (or require vast quantities or unique foods) do not require rest or companionship therefore do not play by the same rules. They have no
needs driving their actions, only
wants. Orcs may do what is perceived as evil only because their goals, values and needs conflict with ours.
In case you were asking, IMO:
Giants aren't innately evil for the same reason, they're just big humanoids.
Undead are by-and-large non-sentient. The few that are (and I stress few) are usually created via black magic that corrupts the mind/body/soul. Some undead may fight against this, but the "cure" for their corruption is usually final death.
Dragons walk the line because they are nearly immortal, but have all the same physical needs as a mortal, just 100 fold. Dragons gravitate toward ideological extremes as they age. As their perspective disconnects from mortality.
I've already gone through and addressed this with all the races in my setting.