I think it's much more arbitrary. Silverlight is on life support with support for EME/HTML5 video on most platforms, but Netflix has historically chosen to only support 1080p video on the first-party browser for any given OS (Chrome on ChromeOS, Safari on macOS, and IE/Edge on Windows).

EDIT: Looking at some stuff, it seems like Netflix might "trust" a first-party browser to select the highest-quality stream that it has hardware video decode support for. In comparison, it sounds like there are extensions that enable 1080p in Chrome by pushing it into the list of playlist options, but it can cause a serious performance hit by decoding on the CPU.