There is some books out and that are coming out that is supposed to fill in the blanks. Supposedly the book of the Force Awakens has some thoughts by Luke. I personally think after killing the padawan and other Jedi, Ben couldn't kill his cousin. He just decides to drop her off on Jakku. That is why she was dropped off with the junk dealer. It is pretty obvious the junk dealer wasn't very good to Rey. Being young she might not have realized Ben killed everyone so she would have wanted to leave with him. It would fit in with him being conflicted between the Light and Dark side. The book mentions that after killing Han he actually feels weaker in the force where he thought he would be more powerful.

If only Ben knew Rey survived it would make sense that no one came to get her. Everyone probably thought she was dead along with every other padawan.