She started her career really young, when being gay was a lot less acceptable then now.
Also, I think it's important to remember that there's a big difference between being secretive and giving a public announcement. The rumors of her being gay were circling around for a while, I don't know if she denied them or not, but it was almost common knowledge. I can't even think of one case in which a public announcement about someone being gay was surprising in any way.
Though I think that it's important that celebrities out themselves in order to support the gay community and become role models for others, i also understand the need of privacy and the fear of being judged for your sexual preferences.
Really, it's nobody's business but her own...