Bella's Lullaby- Why they used Yiruma - River Flows in You.?

In the piano score for the movie Twilight bella's lullaby is the original by Carter Burwell but in the movie the piece used is River flow's in you by Yiruma but they also asked Robert pattinson to try to produce a piece as he has a background in music. I was just wondering if anybody knows why they made the decision not to use the original or the one Robert Pattinson made but chose something completely different? Please explain!