I suspect that there are few writers in Hollywood who have any idea how to write strong female characters.
While that's certainly an element, I think there's also the aspect of the audience being trained all of our lives to accept that male characters are the default "for everyone" setting. Fiction media with mostly female characters often gets dismissed as chick flicks, chick lit, etc. (Dismissed in how it's marketed, dismissed by a male audience because they're not supposed to identify with girls, dismissed by some of the women to whom it's marketed, because we're taught that women's stories are less important.) Plus female characters in male-dominated shows often get measured by a more exacting standard than the male characters.

Interesting link about women writers in Hollywood.

I don't know Straczynski's work, but having looked him up, I'm noticing something he and Whedon have in common--a certain degree of internet savvy. It's an interesting connection, and I'm curious if there's something to that. I know I've learned more online about writing and gender than anywhere else.