Is because the TRA don't like it? Because they realise that everyone knows what transvestite means where as 'transwomen' is much less understood term. It could mean either a man or a woman presenting as the opposite sex (I thought it was the later till recently), it implies they are a subset of woman and implies that they have actually had a 'sex change' op. If we're not using the term because the TRAs do t want us to why are we going along with it - after all they use 'cis' 'terf' etc.