Oh, I don't know, could it be because trans people are still beaten to death for being trans, and they have to be very careful about who they tell unless they want to be killed, beaten up, raped, or harassed?
Or maybe it's because shitty people like you still exist and sometime trans people just get tired of being called "disgusting", so they keep it to themselves(Which, by the way, isn't that a common rhetoric you people use? "I don't mind gays/bisexuals/transpersons, as long as they keep it private and don't flaunt it around in public" Well, she did what you asked!).
Or maybe it's because she's sick and tired of her old body and sex being used as a weapon against her, ruining her present relationships and opportunities.
I don't know, man, take your pick.