User blog comment:Lorchyism23/Joining the Thalmor/@comment-1840877-20121118093101/@comment-5295580-20121204040756

One huge reason why you won't be able to join the Thalmor is after the main questline you are a wanted man/woman to the Thalmor. So since you broke into the embassy killed multiple agents then killed more agents in Riften they don't like you one bit and would kill you if you showed up to join.