Because the "most obvious" case is often a trap and we frequently get followup studies years later debunking them.

Why are you assuming "sexism" is the most obvious explanation?! It could also simply be a communications style miss-match, or any number of things. The article indicates the skills required for males to better treat "such patients" are transferable; do people who are overtly or subconsciously sexist typically grow out of it?

As well, the biases negatively effecting the doctor/patient relationship and subsequent outcomes could be within the patients themselves.

Absolutely, study the sexist angle. However allow me to disagree that it is the "most obvious" explanation.