Because peak brightness varies too much (on mobile especially), and the UHD alliance spec for mobile has literally only been ratified for about three weeks so not all the implementations are consistent yet.
And yes, it also requires per device DRM assessment.
This won't unusual at this point in a new at standard. HLS was much more fragmented for its first year, you literally found implementations for some handsets caused others to hard lock.
So this smart text selection thing on O is amazing. It's like a more active now on tap. All morning I've been going through threads tapping on names and titles and the functionality works almost every time.