Chaosmancer
Legend
On every older WoTC product in the DMS Guild right now:
"We recognize that some of the legacy content available on this website, does not reflect the values of the Dungeon & Dragons franchise today. Some older content may reflect ethnic, racial and gender prejudice that were commonplace in American society at that time. These depictions were wrong then and are wrong today. This content is presented as it was originally created, because to do otherwise would be the same as claiming these prejudices never existed. Dungeons & Dragons teaches that diversity is a strength, and we strive to make our D&D products as welcoming and inclusive as possible. This part of our work will never end."
Well, glad to see that they are taking action.
Simplicity, mostly.
When the enemies are the same creature types as the PCs then said PCs are faced with a choice: either go full-on murderhobo and kill 'em all anyway or start having to sort through them and by whatever means determine which ones are worth saving and which aren't (or failing that, somehow get them all back to town so the local authorities can look after it).
Far simpler just to be able to say "There's Orcs in those woods; and if you meet one in there either you or it ain't coming out." That said, if the PCs go in and meet one and decide they want to talk to it rather than fight it, cool.
Slightly pendantic, but they all do share the same creature type anyways. Humanoid.
And, that being said, a party full of humans encountering a war band of dwarves is of course going to react very differently than if they encountered Orcs right? So, it isn't so much encountering a different race of beings, but it is encountering the ones labeled evil and savage.
And hey, look, I get simplicity. I honestly do. But, you can have simplicity without needing a race of bad guys. A typical orc is CR 1/2. Other low CR enemies that can convey the same simplicity.
Beasts (everything from bears to wolves to giant spiders and crocs)
Cultists of demons or devils
Blights
Low Level demons and Devils
Fungus and other plants
Ghouls, Zombies, Skeletons and other undead
Magmin and Mephits
Scarecrows and other constructs
All I really didn't include was anything with the humanoid tag (excluding the cultists) because I already covered that. You could have elven bandits, dwarven war profiteers, ect ect ect. And having orcs be a playable race does not prevent them from the same stories that those races share.