Politics is a sufficient condition. Perhaps Sauron would not tolerate 'independent' Orcs he can't use as canon fodder?
>Again, the lack of people and trade on Anduin is a shocker for me.
Yeah, it's the humans which are hardest to explain away. Some peasants should have thought 'We're tried of paying taxes to Gondor. We'll just move and no one can or would even bother find us'.
Elves are not only long-lived, but also bound to Arda. When they die, they rest some time in Mandos' halls and then reincarnate. No matter how twisted and shitty the world becomes, they are stuck with it. No matter the traumas they suffer and the wrongs commited by them, they cannot escape it [the consequences]. Because their victims will be around forever too, and they can't forget either.