Surprisingly, AV1 is easier to implement a decoder for than VP9 (by design). Some of the VP9 transforms were axed with AV1 purely to make the stream easier to decode.
Why AV1 hardware decoding has taken so long seems to be an issue with hardware manufactures not wanting to support it. HEVC and VVC seem to have more hardware support.
HEVC was built from the ground up to be implemented in hardware to stem the tide of the open-source codecs and thus keep the license money train running.
Why AV1 hardware decoding has taken so long seems to be an issue with hardware manufactures not wanting to support it. HEVC and VVC seem to have more hardware support.