Most people who go to bars tend to be young. I know that isn't entirely true, but I did say "most".
The lack of transgender bars in NY may be because most younger transgender people go to either "gay" bars or straight bars. A lot of younger people are going in hopes of meeting someone new for a fun night of (wherever that goes). If that's any part of your plan, you go to a place where you stand a reasonable chance of meeting someone you are interested in that might be interested in you.
So, if you are a woman looking for a woman, or a man looking for a man, gay bars probably work as well as anything. If you are looking for a straight person, straight bars are everywhere. If you are going to a transgender bar, there is a good chance that you are looking to meet a transgender person for whatever reason.
I suppose I understand the attraction of a safe place bar. NYC being what it is, maybe many younger transwomen and transmen go to straight bars and clubs where anything is acceptable?