Okay, so everyone is jumping on the Star Wars "Rey is Luke's daughter", "Rey is Han and Leia's daughter", "Rey is a Kenobi" bandwagon and there is every theory on why she's on Jakku. Personally, I don't care if Rey is the kid of anyone famous (and by famous I mean Luke, Leia and Han, or Obi-Wan). But here is a hodgepodge of theories as to why she would be sent away to Jakku.
Now, in the novel Leia tells Han that Snoke knew that Ben would be Force sensitive (How could he not?!) and for whatever reason, she thought it'd be a good idea to send Ben away to Uncle Luke to become a Jedi and be pulled to the light side. As most of us know, Ben is manipulated by Snoke, chooses the Dark Side, and becomes the moody Kylo Ren. Okay. We got that.
Rey is not a Skywalker/Solo/Kenobi
It is assumed that Rey is the kid of Luke or Leia because of her supposed presence at Kylo's rampage on the younglings (among many many other reasons). I think not.
If could very well be that Rey's parents were friends of Han and Leia, perhaps they were politically involved with Leia or knew her from her days as senator of Alderaan. When Rey's parents realized they had a force sensitive child, it could be that they turned to Han and Leia, seeing that the couple had borne a force sensitive child as well.
Here's where it starts to get murky
Here's two different ways this could go: Under the advice of the Solos, Rey's parents sent young Rey to Luke, where he began to train her with the other younglings, and that she was present when Kylo murdered the other younglings. This would explain Rey's vision in the Force Awakens. Psychologically speaking, it's totally reasonable to assume that Rey would block out such a traumatic experience, and thus not remember it fully. After seeing what happened to the other force sensitive children, Rey's parents may have decided to send Rey to Jakku, similar to Luke.
Another way this could have gone is that Kylo killed the younglings before Rey's parents sent her to be trained to be a Jedi, and her parents decided it wasn't worth the risk of their kid turning to the Dark Side or being sliced and diced by an impulsive and irrational kid. So they simply sent Rey away to Jakku, knowing that there was no way that they could control her power without exposing her to the Dark Side. They choose Jakku in hopes that by sending her away to an outer rim planet, no one would associate Rey with her political parents, and never wonder why she's not with Mr. and Mrs. So and so, or hold Rey for ransom.
All of this is speculation, and we won't know until 2017 comes along with Episode 8. There are a lot of holes in this theory, so don't take it too serious.