Let me give you the explanation, blunt and simple. Diablo (lore, games, books and all that) is a franchise made for kids, teenagers and gaming adults. This means the people here are not very bright and are not expecting a logically sound masterpiece a la Dostoyevsky or Shakespeare or whatever other good story.
It's a franchise made for you to kill shit catering to the same kids that like Twilight, Harry Potter. You can get away with shit in order to make it feel "epic" and the player to feel "awesome and important". Whatever that means.
You're assuming every kid, teenager, and "gaming adult" isn't bright? lol... I'm guessing you're one of the three. This is honestly a stupid answer to the OP's question. It has nothing to do with the fact that D3's audience has a "lack of intelligence," or the inability to pick up on plot holes. They just went for a twist in the story without making it completely in-line with the previously released lore. It fit with what they were trying to do with D3's story- so they did it.
You're assuming every kid, teenager, and "gaming adult" isn't bright? lol... I'm guessing you're one of the three. This is honestly a stupid answer to the OP's question. It has nothing to do with the fact that D3's audience has a "lack of intelligence," or the inability to pick up on plot holes. They just went for a twist in the story without making it completely in-line with the previously released lore. It fit with what they were trying to do with D3's story- so they did it.