Why didn't James Earl Jones get voice over credit for Star Wars?

From the IMDB trivia page

James Earl Jones supplied the voice of Darth Vader, but specifically requested that he not be credited. At the time, the reason he cited was that he felt he had not done enough work to get the billing, but he later admitted that he didn't want his name associated with the film because he was still an up-and-coming actor, and didn't want to be typecast. Jones does receive billing in the subsequent sequels and the 1997 "Special Edition".

From an interview with James Earl Jones (2008):

Q: How come you didn’t take a credit for Darth Vader’s voiceover in the first “Star Wars”?

• The first two. You know the story, don’t you?

  When Linda Blair did the girl in “The Exorcist,” they hired Mercedes McCambridge to do the voice of the devil coming out of her. And there was controversy as to whether Mercedes should get credit. I was one who thought no, she was just special effects. (Note: The Screen Actors Guild helped her eventually receive credit for her work.)

  So when it came to Darth Vader, I said, no I’m just special effects. But it became so identified that by the third one, I thought, OK I’ve been denying it, I’ve been saying it sounds like the (7-Up soft drink) Uncola nut guy Holder. Geoffrey Holder! ... and I got away with it.

  But for the third one, I said OK, I’ll let them put my name on it.

Q: People thought it was Geoffrey Holder back then?

• A lot of people I talked to did. I’d say, “Yeah, it does sound like him, doesn’t it?”