Well, the OP wasn't that long. I'll try to keep the reply from being too long.

First of all, let's get our numbers right. The largest survey of trans* people ever done is the National Transgender Discrimination Survey. Fortunately, it contains demographics, including a breakdown of sexual orientation of mtg persons. Survey sez:
Bisexual: 31%
Gay/lesbian/same gender: 29%
Heterosexual: 23%
Queer: 7%
Asexual: 7%
Other: 2%
So the data do not seem to suggest that the majority of transwomen are, in fact, attracted exclusively or predominantly to women. If we were to assume that all the bi people lean toward women, we would get 60%, a majority, leaning in that direction. But there is no basis for making that assumption.
Admittedly, asking this question is complicated by the fact that the answer depends greatly on what gender is viewed as "same" or "opposite." Depending on many variables, including state of transition, among others, these questions can get confusing even for trans* people and may influence the data. However, there is nothing here to suggest exclusive or primary attraction to women.
But what is clear is that "other than straight" is drastically over represented compare to the general population. The Williams Institute estimates that about 3.4% (don't hold me to that because I'm going solely on memory, but I think it's roughly correct) of the general population is LGB. (The oft quoted figure of 10% actually originates from an article in the 1970s or 1980s in which the author speculated that as many as 10% of people might be gay. It has no factual basis). So clearly the transwoman population is vastly more likely than the general public to identify as LGB. So why?
Perhaps part of the answer lies in the fact that most men are straight. Whatever makes people trans* may often not affect a person's sexual orientation. So basically it operates such that instead of a cisgender male, the person is a transwoman. But they still have the mental/neurological equipment for sexual orientation to be a straight man I f they were not trans*. And since they are trans*, but they still have the same sexual orientation as a straight man, they are attracted to women. I hope that made sense.
But let's build on this a bit more. Not everyone experiences sexuality as being about one gender or the other. For example, for some people, the experience is more about "sameness" or "differentness." For those people, if they live as male and are attracted to women, they may find the attraction switches as they go through transition. Why? Because women are attractive to such a person because they are different rather than because of being women. The more this person goes through transition, the more women become "the same" and lose their attractiveness. To the extent that sexual orientation has any meaning for me, I'm like this by the way. "Different" is what attracts me, so my gender preference switched in transition. A person whose attraction is to "the same" would experience a shift in their attraction for the same reason.
Those are some thoughts off the top of my head. I think what it comes down to is "it's complicated."