I feel it's because we seem to have to pretend to forget (or made to pretend) that we had, historically, 2 groups, transsexuals and transvestites. Both of those groups were predominantly male, but not the same. They behaved very differently, and were treated by the society accordingly.
The laws that were explicitly written, in the UK, for transsexuals, were interpreted to apply to the new term 'trans', which is an umbrella term. Which includes transvestites, explicitly - a term EL previously used to accurately describe himself, before his conversion to 'womanhood' 