This is a topic of a front-of-the-book article in the October issue of Philadelphia magazine, and after reading it, I believe the author--and Philadelphia's readers -- would have been better served if they had read one of Michelangelo Signorile's explanations of why some politicians
deserved to be outed and others not in the Advocate.
As it is, the article is competent enough: the point it makes is that nobody in the Trenton press corps felt that McGreevey's being gay was story enough in and of itself to devote throwing investigative resources at in a big way. Reporters did repeatedly ask the governor about rumors he was gay, which were routinely denied, and when the news of Golan Cipel's appointment to a post he was apparently not qualified for broke, the angle became more relevant and was pursued by more media outlets. But still nothing appeared in print or on air stating definitively that Gov. McGreevey was gay, and nothing would have had McGreevey not outed himself in his resignation announcement.
Of course, McGreevey outed himself mainly as a means of diverting attention from yet another unsavory tale of corruption; such tales have been staples of his term in office. But nothing else he did made his being gay relevant to anything else.
Which brings me to the reason why Signorile might have been a better person to answer that question. As the article fails to note, just about every politician who has been outed was kicked out of the closet by the gay press or gay activists bent on exposing their hypocrisy in supporting legislation or other state action that was hostile to the notion of equal treatment for gays. As McGreevey did no such thing, there was no real reason to reveal his sexuality against his wishes except in the context of some scandal where it might have influenced his decisions, as was the case with the Cipel appointment.