Patent complications do seem to be a factor, but it's unclear if in practice it'd be "loads of money", as the project page suggests many existing systems already have licensed decoding capabilities.

Also, one of the patent pools covering HEVC announced in 2016 that software-only implementations would be royalty-free:

https://www.prnewswire.com/news-releases/hevc-advance-announ...

Whether still-other licenses from other patent-holders might be necessary for software-only, in-browser decoding is unfortunately murky.