I think these days it is considered politically incorrect in some circles to use a female form of the noun. In some cases a separate, unisex term has arisen, for example air hostess becoming the unisex flight attendant. I think it is connected with the fact that English rarely marks gender - if we said someone was tall or intelligent, no one would know whether that person was female or male, so why should their job description give away their gender? I suppose it's the idea that having a separate word for the female worker indicates that she is not doing the same or as valuable a job as the male worker.
Personally, I don't think that at all and I am perfectly happy to use waitress and so on, but I believe that's the reasoning.
Acting is such a competitive profession, the ultimate accolade, being worldwide recognition of your work. It is deemed more professional to have a generic form of the noun in an industry where women play men, and vice versa. Wages for actors are not biased towards gender, but the success of the individual. The gender-specific, "actress" is quite appropriate when discussing individual actors, but when referring to them collectively, as in "the cast" - of a film for example, "actors" is more correct. To say, "She (Helen Mirren) is an actor" may grate with some people, but it's not incorrect. To say, "The actress, Helen Mirren" is correct too, because it introduces the gender of the actor in conversation and text.