Question: Why did they use actors who couldn't sing in the leading roles? Wouldn't it be easier to cast people who could sing instead of dubbing with other singing voices?
Answer: Natalie Wood was more well-known. And Wood actually COULD sing, she just couldn't hit the high notes perfectly. In fact, she was told only some of her singing would be dubbed.
Answer: People would not come to the theater to see Marnie Nixon despite the fact that she has sung the lead roles in West Side Story, The King and I, and My Fair Lady. They'd rather see that darling girl from "Miracle on 34th Street" again (or Deborah Kerr or Audrey Hepburn) than an unknown with actual talent.
Myridon