Well, its hard to make a valid determination on why this is because I ain't a part of the Hollywood structure. However, I have some armchair theories with no academic backing that could supply some semblance of an answer:

• Hollywood is progressive. By showing transgender women of color, it queues people in on the idea that transgender issues are not separated by race (ie, anyone can be transgender). It shows that transgender people are indeed people and that there's no underlying, false correlation of environment, thus codifying that transgenderism is a circumstance of genetics. It also walks hand-in-hand with intersectionality, a movement that seeks to make connections between various struggles of societal acceptance.

• Hollywood is racist and trans-phobic. By using women of color as opposed to the majority standard of Caucasian, it underscores these people as "other" and different from mainstream American society. A sort of entertainment industry short hand for "these people are different!" Personally, I feel this is the least likely answer.

• There simply isn't enough transgender voices in media with a high level of visibility. Transgenderism hasn't been discussed in a broad context for sometime, gaining only the slightest bit of acceptance in the 90s when RuPaul came onto the scene. (Additionally, is RuPaul even transgender or is RuPaul just a crossdresser? This is an important distinction that needs to be made.) Even since then, I can only think of one additional transgender voice in American media that has had a high level of visibility. The fact that well, both of them are black would speak less about how we view race and more so about how we view transgender people, which in this case is rarely and only for the past 20 years or so.