I have one team moving over to GitHub right now.
The power platform community has generally been slow to adopt this one (I think) because GitHub work item management has historically been such a letdown next to DevOps. No boards, no epics, no features... For a long time moving to GitHub would have meant rolling backwards in capabilities, PLUS, most pro dev teams have already made their MSDN / VS Enterprise investments and GitHub is a different license altogether. So when you put those together, it is pretty easy to see: from the perspective of a decision maker, moving from DevOps to GitHub would mean spending new money and a bunch of angry scrum masters. Then, in case that wasn't enough, migrating DevOps code is easy (Git to Git to win!), but moving the work items is a genuine nightmare if you're done any process customization at all (which everyone does). The interactive experience DevOps gives for work items is great, but the data migration is downright Lovecraftian.
Recently, however, GitHub has made huge strides; adding boards and extending the customizability of work items to where the feature gaps are not really painful anymore, and at the same time it has rocketed past ADO with static code analysis and dependency monitoring and a host of other capabilities. So teams are making the switch (myself included). Sadly, the migration nightmare of DevOps will continue to discourage lots of folks, so the transition will still take a very long time.
