It's DRM. That's why you see 4k available in edge (where they use special CPU instructions for secure enclave - that's why requirement of newer i7 CPU) while 720p only in Chrome.

I think it has to do with HEVC h.265 codec & HW acceleration support available to the browser in the respective platforms.

https://caniuse.com/#feat=hevc

Not sure why Chrome doesn't support HW for it though albeit Chrome OS. My Celeron N3060 Chromebook can decode it.

I read Netflix has started using VP9 for streaming, newer CPU's have VP9 decoders in them.

No it's actually a DRM limitation. You're not allowed to watch high definition content of your platform doesn't support latest Widewine DRM even if you have the HW or SW decoding capability.

Yeah, IE and Edge are the only browsers with PlayReady DRM. Rest of them (besides Apple devices) are using Widevine DRM.