IT was just how they made movies back then. One also sees it in men's hair styles in films and TV made in the seventies. I think part of it was that producers didn't think it made a difference and another is that actors and actresses were very reluctant to change their hair styles as they needed their 60's/70's hair in other roles. If the choice is having a name actress in a leading role with 60's hair or some no-name, less talented woman, they'll go for the 60's hair. Similarly, if all the actors showing up for a casting call for a MASH episode have longer hair than any Korean War era man would have, they're going to suck it up and go with it rather than have no one at all.