Its about percentages. Approximately one percent of the population in the US identify as trans. Then pare that down again by the percentage of those people who can act and have the desire to do so. Do it again for people who are age appropriate for the roles being cast, and then do it again for every role. Even when you start with a very large number, that many filters is going to cut it down fast. Think about the stars that need to align for a transgender person to be in or able to travel to LA, being the right age, having the right look, is able to work with others, has the capability to act, and manages to meet the right people to get in touch with this production.

I'd honestly be shocked if they found more than a handful, let alone a full movie cast.