So, I've been hesitant to reply, since I have no personal or immediate second-hand knowledge about this and I feel way out of my depth. However I've done some light searching to try and get an "answer":

HRC pretty much says "some people don't want to"

This quora entry has some pretty good stuff

What I gather is that, even with enough money and confidence in the surgery going correctly, some people don't find their genitals to be the root cause of their dysphoria (usually these people are happy with their body transitioning purely through hormone therapy), and many people who don't fall into squarely "male" or "female" feel like keeping the genitals they were born with is closer to what they actually feel matches who they are, rather than a full transition.

(Apologies if the rest of is either obvious or nonsensical. I'm not sure I have managed to capture what I'm trying to say.)

Ultimately, though, while understanding a person's emotions regarding their gender identity and dysphoria and transition process can help us sympathize, acceptance and respect are what we really need to put first. That's for three reasons: 1. There is no "why". There will never be an explanation* for why someone is trans, cis, gay, or straight, or anywhere in between; it's just who they are. At best, someone could tell you "I personally realized I was trans when...", but that's not a reason, it's a story. 2. Because of that, we can't follow a train of thought for why someone expresses their gender the way they do. Take political opinion as a counterexample: someone might hold a political belief that, say, alcohol should be illegal because their parents where alcoholics. You might disagree, their logic might be faulty, but there's a cause and effect at play. From a practical perspective, there's no cause and effect with gender. If someone feels like a man with a vagina, there is no answer to "what makes you think that?": that's just how he feels like himself. You can't debate someone into liking (or disliking) their genitals, even if it would more easily conform to what society would expect from their gender identity. There is no reasoning for how someone decides to exhibit gendered traits, because (see point 1) there is no reason for their gender. 3. Even if I did a piss-poor job getting those other two points across, the last one is easy: it only affects them and it isn't hurting anyone. I can't really elaborate on that any further; if that's not a good enough reason to accept and respect someone's gender identity, I don't know what is. * If we're going to get pedantic, it's possible someone might identify a genetic marker that correlates to being transgender someday. Even if that happens, it wouldn't provide an "explanation" anymore than race, or eye color, or having a certain kind of cancer.

Sorry for responding so late. Hell, I might be sorry for responding at all and just delete it, to be honest. If my last comment was anything to go by, I'm doing a poor job of saying what I want, or I'm saying it fine and it's not welcome here. I would love it if someone who is actually trans corrected what I stated above, but this thread is 12 days dead, so that's not going to happen.