Their official response is "because not all browsers support it".
It could be a valid business decision (I.e. uneven browser support will confuse our users and increase costs) but I think they are just using that as a delay tactic.
Stable API or not, every time there is a new NVIDIA driver, my Win10 boxes work and my Fedora/Ubuntu ones break. I doubt Microsoft is asking NVIDIA to keep their driver inside Microsoft repositories.
It could be a valid business decision (I.e. uneven browser support will confuse our users and increase costs) but I think they are just using that as a delay tactic.