The gates to Oblivion were closed. All the gates.
And Skyrim already has incredibly harsh policies about Daedra worship (as you may have gathered from the roaming patrols of people trying to kill Daedra-worshippers. So there aren't going to be a lot of Daedra just roaming around.
That svcks, but at least we could have some other non-deadra fantasy monsters. Ogres, Dreughs, Liches, Netches, Boars, Bonelords, Goblins, Flesh Atronachs, Minotaurs, the Udyrfrykte, and Unicorns are all non-daedra who have been present in previous games but not in this one.
Goblins I could understand because their role has kinda been replaced by the Falmer, but I feel like in the south near Riften they could've made some appearances.
The others though have no reason for being left out. Idk if Netches are native to Morrowind/Vvardenfell and not found anywhere else, but I excluded all other things that are known to be native to only a certain place. Oblivion, Morrowind, and Daggerfall all took place in provinces bordering Skyrim (albeit Morrowind was on the island of Vvardenfell, most of the species and monsters should be similar if not the same afaik), so there's no reason that some of the monsters should just disappear.