[cubefox]

Unfortunately AV1 hardware acceleration support was really slow. Apple only just adopted it in their new SoCs. Qualcomm didn't support it until recently either. It's probably still years till the hardware support is anywhere close to H.265, or even H.264 (the currently most common format). It's possible to decode AV1 in software, but the computation requirements are quite high.

[lern_too_spel]

HEVC is from the VP9 generation of codecs. Qualcomm has supported VP9 decoding since 2016 (Snapdragon 820). Unfortunately, no Apple device has ever supported hardware accelerated VP9 decode, so HEVC is the only UHD option on those devices.

[mort96]

I guess it doesn't really matter anyway since AV1 is also patent encumbered...