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.

This is a misapplication of Occam's Razor. If a study discovers a correlation between two variables, the Razor does not take the side of either causal direction.

In this case, the "simplest" explanation as far as the Razor is concerned is not that doctor and patient's gender influences outcomes, and not that probability of some outcome influences the gender of the chosen doctor. I believe (somebody please correct me if I'm wrong) the side the Razor takes is that there is no direct causal relationship at all, which is why the burden of proof is traditionally on the experimenter to demonstrate that such a relationship does exist.

That is not the most obvious explanation from my perspective. A more "obvious" perspective to me is that female doctors are better at treating female patients because they possess a better understanding of them. I believe that a big part of managing post-heart attack patients has to do with lifestyle changes. It seems very plausible that a female doctor could do a better job (on average) when helping their female patients in that area of their lives.

I agree that we should investigate it, but without that investigation, Occam's razor becomes more of an undiscerning broadsword.