Speaking personally (is there any other way..?) three principal reasons:
1) The general stigma and misunderstanding associated with CD/TG folk
2) The potential pain and misunderstanding that may be undergone by close family, specifically my wife (who has enough to deal with)
3) The impact of that on my career and potential job prospects as well as charitable work undertaken for the local community
Secret Drawer said it right for me when she said it may be better in some ways to be honest, but it is not necessarily the right thing to do, depending on circumstances.
And you too MsVal - you are right to highlight that we each as individuals have to make a choice and live with that choice. It is ours and ours alone and we live with the potential consequences both ways.
I don't think I would have revealed as much as I have on this site if I was still pursuing an active career - but then the need that I feel can be satisfied with just a few hours a week... For others where the drive is much stronger the decision to supress that is much more difficult and I truly feel for those in that situation.
Katey x