I'll throw in another wrinkle that just came to me.

The Force wants "balance". You had gazillions of Jedi and two Sith (Sidious and Maul, Sidious and Dooku). Then, you had balance, with Kenobi and Yoda on one side and Palpatine and Vader on the other. In New Hope, you lose Kenobi and gain Skywalker. In ROTJ, Yoda Sidious Vader Skywalker. ("There Can Be Only One") He didn't get the full reject-the-Dark-Side training, either. All the balance, the Light and the Dark, would be in him.