Why didn't Julie Andrews star in the Movie version of My Fair Lady?

I love Audrey Hepburn and Julie Andrews but when I searched 'Wouldn't
it be Loverly' all this stuff popped up on youtube. I hadn't realized
that My Fair Lady had been on Broadway. I listened to Julie's version
and it was really nice. Why wasn't she asked to do the movie? Was she
busy or something.

Also, apparently Marni Nixon is the one singing in the movie. Did
Audrey sing at all?