I think the reasons are almost entirely cultural in nature:
1. FtM TS's are quite invisible. Indeed invisible to the point where it's a serious problem for them in many vital areas, such as healthcare. This invisibility contributes to having even worse statistics about them than we do about MtFs. We just don't know. There are some statistics that suggest for people who transition the numbers are about even, but in my area, the ratio is about 3:1 MtF's to FtM's.
2. It's more or less not possible for someone assigned female at birth to crossdress. It just isn't. It generally takes a lot of effort to pass as a man, for most FAAB, at least until the ones who transition have been on T for a while. Most people, observing the FtM, won't even realize that they are even trying to pass as a man. On the other hand, for comparison, put a guy in a dress, send him walking down the street, and everyone knows something's going on with him.
3. Women emulating masculinity in some ways has been a part of feminism since the 70's, and is required, really, to compete in many workplaces.
4. A FTM can, because of the relatively high degree of cultural freedom to express their gender that women have, discover "OMG - I've been actually living as a man for years! I - I guess I am one!" In my experience very few MtF's have this experience. The ones who do are generally CDs in unusually accepting situations socially, relationships, employment, etc. (Betty Crow is an example of this in some ways, I think.)
@Alice Torn:
Trust me, it's not about men feeling "obsolete." The guys who run things feel large and in charge, I can assure you of that.
I'm sorry you don't feel like much of a man. Have you ever considered that you may well not be one at all?
At the idea that FtM's somehow have it easier. I don't buy this. It's different because of physical medicine and social constraints. On the one hand, T does a lot for many FtM's - hair, voice, physical appearance. This is more than what estradiol does for MtF's. On the other hand, the options for bottom surgery are much better developed for MtF's, than FtM's. I mean much better developed that many FtM's choose not to have surgery not because they don't need it - but because the 40% chance that they'll never have another orgasm again after they have it dissuades them. (Well that, and the $100K+ price tag that some of the surgical options require is simply totally beyond their financial means.)
Can you imagine, in our culture, how hard it must be, psychologically, to be just one hell of a man, but to have no penis to speak of, or a very, very, very small one, coupled with a vagina? Look - what I have between my legs freaks me the hell out. It's wrong, and I have nightmares about it. It doesn't define me as a woman though.
There is a lot less emphasis placed on the attributes associated with a woman's genitals than with a man's. Basically, if we have one, we're golden. This isn't entirely true, as any woman who can't conceive a child will attest. It's also different in that society feels it has a proprietary interest in controlling what we do with our vaginas - whereas for men, telling them what to do with their dicks will almost certainly send most dudes to war. In our culture, in many ways, for men, it's all about the dick. (I think this has to be even worse for the FtM's who identify as gay men. My god, I can't even imagine some of the things they must experience, because if you think straight men objectify women, you ain't seen nothin' till you've watched a gay man objectify another man.)