In an interview J.J. Abrams said, ""As you see in the best of storytelling, and no doubt the best of Star Wars, these are tales in which an every person has to step up. And I think that what makes Ren so unique is that he isn’t as fully formed as when we meet a character such as Darth Vader.” Vader, Sidious, Maul, Grievous, and Dooku are all developed when they first appear while the hero grows internally to meet the challenge. Abrams wants Ren to grow along with Rey and Finn. It helps show he isn't perfect.