I wonder if the issue is liability- people suffering from various ailments are liable to Google their symptoms in hopes of finding an at-home cure instead of going to a doctor[0], and if this turns out badly they could conceivably turn around and try to sue Google for providing harmful medical advice[1]. De-ranking sites that provide medical advice, which aren't whitelisted well-known sites like Mayo Clinic or WebMD, might prevent this- or at least provide a better defensive position in the courtroom.

[0] Especially likely in countries with badly-structured healthcare systems, such as the U.S.

[1] Especially likely in countries with badly-structured legal incentives, such as the U.S.