I'm not entirely sure. I believe it has to do with the DRM implementation. While Widewine is the DRM in Chrome and Firefox, Edge uses something different[1].

I've seen people talk about Chrome being able to handle 1080p in ChromeOS. So there seems to be more than just Netflix's choice involved here. Although I bet Microsoft being proactive helped. Definitely unclear.

Also I don't see any reason why Netflix wouldn't want to support 1080p in all the major browsers. The only downside might be excess bandwidth, but with the Open Connect Appliance being a thing, I doubt it is a big factor.

[1]: https://blogs.windows.com/windowsexperience/2016/07/13/get-b...