She's still in love with him because she allowed her happiness to become dependent on another individual. Sakura is a classic example of the popular girl who thinks there's more to her than there actually is. That's not to say there's nothing to her, quite the opposite, but Sasuke's lack of a response to her triggered her obvious desire for attention as the top b*tch of the class. She was attracted to him, of course, and not forward enough to do really do anything about it, and that morphed into obsession. Couple that with life or death scenarios, constantly being in contact with each other, and Sasuke actually growing as a person while they were a team, and it's easy to see why she fell in love with him. What's not easy to see, due to the terrible job that has been done with her as a character, is that she sat back and watched as Naruto grew faster than anyone thought possible, and Sasuke became more and more broken as a result of both Naruto's ascension and his circumstances. Sakura misunderstood their progress and her own lack of progress, and tied Sasuke's decision to leave the village to her lack of progress. Her emotions are twisted into her reasons for training under Tsunade and she's become unable to resolve them because they're her entire reason for being where she is. This would be more obvious if she had actual character growth instead of just dancing the robot from character point to character point, and going years of chapters between actually having an impact on the story.
Now, if we're looking for a reason as to why she's still in love with him outside the narrative, well, I have an answer for that too. I don't recall which movie, as I don't bother with the anime or the OVA movies... might be the Road to Ninja one, dunno, but from what people have told me there's one where she learns firsthand how miserable Naruto's existence is. How lonely he is, how he has very few people to turn to emotionally, how miserable he truly is deep down. What does she do? She makes a joke, then runs off happily with her parents. Doesn't think "Gee, maybe I should go hang out with him sometime, he's my friend.", just goes on about her business. This is why Sakura is still in love with Sasuke, as Kishimoto seems to be obsessed with a state of static emotions.
Characters, save for Naruto, Sasuke, Neji, Lee and Hinata, do not fluidly develop as character. For her part things happen to Sakura, and she remains the same. She should develop as a character, and in a few cases she does, but for the most part that happens off screen. After Sasuke nearly kills her after the Kage Summit, we don't see the aftermath. When she sees Hinata's love for Naruto result in a near death on Hinata's part, she doesn't encourage Naruto or Hinata, she just grumbles about Hinata's sacrifice, and then vanishes until she's needed again. Sakura's state changes off screen if it changes at all, and since we can go months without seeing her, and years without her doing anything of value, we're left with a character who has not had enough moments to realistically develop past her emotional baggage.
tldr; Her emotions are tied up in her reasons to grow as a ninja, and she doesn't really grow as a person as a result of a misunderstanding of her relationships with Sasuke and Naruto. Also, Kishi has selective stupidity when it comes to character development.