This is a pretty interesting topic that I don't think will ever really be fully determined to have a root cause. I think there are various arguments that seem to make a good explanation but they don't necessarily have the same root cause.
As males tend to be driven more by visual stimulus when finding a 'mate' heterosexual men will look for prominent feminine features.. so wide 'child-bearing' hips, large breasts and a curvaceous body to name a few. According to scientific study these curvaceous features determine how productive a woman would be at producing a child. Support suggests that men are attracted to women with these prominent features on a primitive level because as humans our basic design is to reproduce then, live long enough to raise that offspring. It could argued that as transgendered women often accentuate these feminine features to appear more visually female that by definition they are more attractive than genetic women.
To use an example I would argue that 8 out of 10 straight men if asked would say that Amber is very attractive. If these men where then told that Amber was transgendered their conditioning to what they may feel is socially acceptable would change their initial response but on a primitive level they would still find her attractive. Using this argument I'd say all straight men on a primitive level could be attracted to transgendered women as they visually present identically to genetic females with the possible exception of genitals.
Another argument which, I have heard discussed before is that men attracted to transgendered women are attracted to women physically but not emotionally due to lack of understanding. It is argued that men are 'put-off' by female behaviour and don't interpret their brain chemistry. This argument kind of goes hand in hand with Women are from Venus, Men are from Mars. There is a video on youtube which, I'll try to locate and list in this post that suggests in some cases men are more likely to be attracted to transgendered women because they are more likely to understand their wants, needs and desires especially during sex. As there are more visual traits to when a genetic male is about to reach orgasm the video argues that straight men may enjoy sex more as they can tell when their partner is enjoying it. Also the 'straight' man is more confident his partner will more likely be able to meet his sexual desires as her desires will be a lot more similar than a genetic females would be.
I agree with this to a point however in my opinion a transgendered woman's biology is a lot different to a male's more so when this comes to the biological and chemical makeup of the brain. There have been studies where post mortem brain slices of transgendered women have been found to be near identical to a genetic females brain where normally (if male) there would be striking differences. This evidence doesn't take in to account the effects of HRT on the brain but if a transgendered females brain where no different to a male brain Pre HRT then surely we would all want to become trans..
Personally I think the attraction is down to trans women often being more gracefully female and in some cases have a better understanding of the male mind. This is of course not always the case. I have girlfriends who understand me and some who do not
I suppose it is somewhat a mystery.. perhaps for sake of argument being species with a higher cognitive function us humans are just attracted to each other full stop.. male or female it doesn't really matter ;)
http://www.youtube.com/watch?v=p-A8GvUehq4