It wasn't really all that slow in general, just slow on dedicated streaming hardware.

Basically, it was the push to 4K (and especially HDR) that caused HEVC to roll-out. In 2016 4K Blu-rays started coming out and they were all HEVC 10-bit encoded. It took a couple more years before dedicated streaming devices and lower-end smart TVs bothered to start including HEVC support as standard because at first 4K content was uncommon and the hardware came at a premium.

Now that it's mostly the de-facto standard, we see HEVC support in basically all streaming devices and smart TVs.

AV1 didn't have any sort of resolution change or video standard change to help push it out the way HEVC did, so it's basically rolling out as the parts get cheaper due to pressure from streaming giants like Google and Netflix rather than due to a bottom-up market demand for 4K support.

I didn't know about Blu-Ray being relatively prompt. But I still think HEVC adoption was slow in broadcast TV, which I would have thought was a shoo-in market.