if Luke skywalker abandoned his daughter in the hands of Simon Pegg with no intention of ever coming back for her, then I give up. I can't get behind that. what self-respecting Parent abandons his toddler, AT THE MALL, much less, on a desert planet surrounded by aliens that don't care about her?
I see only 2 possibilities:
1). Luke left his daughter with Simon-Pegg-in-a-rubber-suit, with the intention of returning for her, WITHIN DAYS. he ran into trouble. he got stranded on the Jedi Temple Planet, with no ship / no hyperdrive. he had no way of leaving the planet. he had left a trail of breadcrumbs in R2, in case he ran into trouble, but nobody was ever smart enough to figure it out and come rescue him. until now.
2). Kylo left her on Jakku. he was told by Snoke to kill her, along with any other jedi younglings in Luke's care.. but he couldn't bring himself to kill his own family/sister/cousin/whatever. instead he left her on the garbage planet as a way of discarding her, taking her out of the game, without actually bringing himself to kill her. this is his weakness: he is "tempted by the light side". this is why he had to kill Han: to redeem himself in the eyes of the supreme leader. (after failing to kill the girl, many years ago).
kylo knows something about "the girl from jakku" -- he reacted strongly when she was brought to his attention -- he knows something about her. this much is obvious.
(he may be the one who left her on jakku in the first place , as a way of hiding his 'light side tendencies' from his supreme leader, he was tasked with killing her, but at the time he was "too weak" to kill her --- now he's not anymore, as witnessed by what he did to Han)