Board Thread:Lore Discussion/@comment-96.252.245.207-20130116060134/@comment-10207438-20131001050350

Because the reason those races became that way is because of the gods they followed.

The Orsimer where Altmer or Aldmer at one point, their connection to Trinimac forced their change with him. The Chimer changed into Dunmer through the power of Azura acting on their faith in the Tribunal.

The Falmer(changed ones) don't follow any gods that we know of, they have no faith and are therefore unaffected by the gods. It would take such a powerful outside force to change them, not just physically like the other two races, but mentally too, something that no Daedra/Aedra has ever done.