There are those who refuse to recognize the existence of the struggle that is neither with gender–that which society imposes–nor with sexual orientation–they little understand the struggle with the sex of our own bodies.
This continued refusal to recognize transsexual people takes some, obviously absent, understanding. An understanding also absent when one sees that same-sex sex, unsurprisingly to me at least, is gender non-normative, that is, transgender; this is quite different from those whose quest is for body congruity.
So, analytically, LGBT has become gay/lesbian people, and those people who, like gay/lesbian people, are gender non-normative, challenging what society imposes.
The dominant discourse is to exclude transsexual people because our lives are incomprehensible: our challenge is not to societally imposed gender norms, but to the physical dissonance of morphological sex; our challenge is to our own bodies.
LGBT=sexual orientation and consequential challenge to gender norms
transsexual=physical dissonance with anatomical/morphological sex, a challenge to our body’s sex
This might seem to be a challenge to society’s imposed gender norms, but only for an analysis that lacks both depth and sincerity. It is so obviously inconvenient even to say the word–transsexual–let alone demonstrate understanding of it; it is something quite different from sexual orientation and gender non-conformity.
UPDATE: I have recently come across this report purporting to be about GLBT health in Washington D.C. which does not include “transgender, transsexual and gender diverse” people.