On Windows Microsoft/Nvidia/AMD sign their graphics drivers, which guarantees the DRM that the content isn’t recorded on the system.
Disclaimer: The following is my understanding from reading things here and there. I’m a layman on this topic, so please don’t quote me.
On Linux drivers aren’t designed to prevent users from recording on their system, so the DRM doesn’t play high quality content. Also, because drivers aren’t directly provided and signed by MS/NV/AMD, there’d be no way to prevent users from patching the graphics drivers to allow recording again.
That is, if DRM support was implemented in the driver, which it won’t, because there’s no interest and the current distribution model makes it near impossible.
tl;dr
DRM is (always?) closed-source, else it could be easily circumvented. The Linux driver/desktop stack isn’t designed to prevent users from accessing content played on their own device, so rightsholders disallow playing high quality content on Linux.
PS: I’ve noticed on Amazon or Netflix some shows are higher quality than others on Linux. I guess this might be due to rightsholders requiring different Widevine levels for the same quality.