It's all open source software. Stupidly simple and easy to host. It's a low value commodity thing without mucb value that anyone can trivially self host. All you need is a docker capable machine (any linux machine basically) and some disk space to host the images. And a bit of operational stuff like monitoring, backups, etc. So there's an argument to be made for using something that's there, convenient, and available but not too costly. Which until recently was dockerhub. But apparently they are happy to self destruct and leave that to others.
They should take a good look at Github. If only for the simple reason that it's a natural successor to what they are offering (a free hub to host software for the world). Github actually has a container registry (see above for why). And of course the vast majority of software projects already uses them for storing their source files. And they have github actions for building the docker images from those source files. Unlike dockerhub, it's a complete and fully integrated solution. And they are being very clever about their pricing. Which is mostly free and subsidized by paid features relevant to those who get the most value out of them.
I like free stuff of course. But I should point out that I was actually a paying Github user before they changed their pricing to be essentially free (for small companies and teams). I love that of course but I was paying for that before and I think they were worth the money. And yes, it was my call and I made that call at the time.
Also worth pointing out that Github actions builds on top of the whole docker eco system. It's a valuable service that is built on top of Docker. Hosting the docker images is the least valuable thing. And it's the only thing dockerhub was ever good for. Not anymore apparently. Unlike dockerhub, Github figured out how to create value here.