We've wanted to make this change for the last 18 months, but needed our Enterprise business to be big enough to enable the free use of GitHub by the rest of the world. I'm happy to say that it's grown dramatically in the last year, and so we're able to make GitHub free for teams that don't need Enterprise features.
We also retained our Team pricing plan for people who need email support (and a couple of other features like code owners).
In general we think that every developer on earth should be able to use GitHub for their work, and so it is great to remove price as a barrier.
OAuth is way more complex, I've used it countless times and still get confused by it. It has more complex patterns like having a separate resource server and authentication server, it's used for more purposes, e.g. sometimes for API access and sometimes for login and sometimes a confusing mix of both, and there are big differences between v1 and v2 and some services are still using v1.
I once tried to implement it, and found that the specification was spread across ~500 pages of dense PDFs. I find it to be complex.