Sometimes it's dependent on finance- it is for me, anyway. Say your career is being fickle, or even the fact that you are trying to make money to pay for hormones, therapy, surgery, etc... If you are not in the position to do so, that can be a very strong halt. Not that surgery is the major thing about transitioning, but for me it's vital, otherwise I don't see the point if I can't do it the way I want to. There was no transitioning for me if I couldn't allow myself that, not even mental/emotional.
Finance was the pre-cursor for me. I know that people have worked around finance, have insurance, just being resourceful- I couldn't do that. I only felt it was time when I didn't have to worry. I still do worry but that's just because worrying is a past time for me.
I've felt this way since I was a child, it only made sense for me to pursue it when it's feasible, because I've waited this long anyway.