- Patent situation. If you're a camera or phone maker, it is quite a risk to use HEIF. Even if you pay all the major patent pools, you only cover part of the patent claims. You'll need dedicated hardware, so once you've produced the devices, the patent trolls have quite some power since you can't just switch to another codec with a software update.
- The codec is not optimized for high-quality still images – it's a video codec, and has its sweet spot in the lower bitrate regions. Photographers care about subtle texture, high bit depth, etc.
- There's no progressive decoding – again, it's a video codec, where progressive decode of a single frame is not useful, but for still images on the web, progressive can be a desirable option.
Yes but it is submitted as an open standard to an Organisation that doesn't run or belong to Google.
Is HEIF used anywhere other than on Apple devices?
A bigger problem is that various companies just did a lot of work to adopt HEIF so they're not going to be eager to churn onto JPEG XL. Generally people wait 5-10 years before they're open to upgrading.