Anakin wasnt the Chosen One. Luke was, and Luke brought balance to the Force.

edit: Kishi has a good point though. The Jedi assumed balance would be in their favor, but whose to say whether it is or not.

Luke was taken in by his Aunt and Uncle, so there was no reason to say they were his parents. He was with family, while Leia was not, and Bail chose to raise her as his own. It would probably be less suspicious this way.

Obi-Wan won at the end because he finally gained the upper hand. He didnt hav ethe upper hand the entire time. Plus, he may have been holding back. He cared for Anakin very much, so it was hard for him to strike down his friend.