https://ngcodec.com/news/2017/10/21/why-we-are-supporting-vp...
The simplicity of VP9's licensing versus the complexity HEVC's licensing probably has a lot to do with that.
Meanwhile, nothing iOS supports VP9 HW decode. Until they come around, people could give a fuck about installed base.
Then there is the stuff this article doesn't very well measure. Like DVB-T2 in Germany using 1080p50 10 bit HEVC. That is expected to be around for a decade or more. That is a whole lot of TV settop boxes.
The amount of Android devices dwarfs iOS devices and Apple knows that it is not viable position in long term. That's why they joined AV1. People in near future would not give a fuck about the Apple, if they couldn't play their Youtube or Netflix videos in 4K on them.