Back in the 1980s (there abouts) LG organizations started to become political. They wanted to achieve a lot of what you see happening today. Mostly, it was because so many were getting AIDS and their partner was being denied access to them. In other words, if you came down with AIDS, your parents could deny access to your gay partner. The relationship was not valid. You as a partner, had no rights. Say there were children involved. You want your partner to become the legal guardian should something happen to you. Your family could over rule that decision and not allow your partner to ever see the children again. Say you had a 401k and your partner was the beneficiary if you died. Not happening, your parents got the money instead.
So because of sheer numbers, the B and T movement was rather small but were substantial enough to grow the ranks of the LGBT movement. Instead of reading like the alphabet, T was chosen to include crossdressers, transvestites, Transsexuals etc... As more sub cultures arose they were included under the T designation.
Now while you may think you and your brand of T is special, try going to congress and explaining why all 14 people like you should have your own special rights. You will be laughed off of Capitol Hill.
So that is why and how LG became B and T
