For big projects like Linux or Python, they don't move to Github because they have their own system running for years. They just don't see the benefit and they find their existing mechanism effective. Many projects today still run on Bugzilla, mailing list and IRC.
For smaller projects the cost to migrate from older services like Google Code or even from Bitbucket to Github (or sometimes from Google Code to Bitbucket) is not trivia. I think one reason why Django still use its own system (Trac) rather than going for full Github is the difficulty to migrate old issues over.
So if the reason is "because we have a solid process in place" it is understandable. But actually, running your own system is also a lock-in but you just happen have full control of the system. You are lock in because you are still using that one software which may or may not continue to exist in a few years. If next decade you decided you don't want mailman to run your mailing list, instead you want to use MailFooBar, you will need to prepare migration. The only advantage is you control your stack, your data. Github can shut down next year all the sudden and you can lose everything (this is particularly true and dangerous for people whose software development cycle depends on small start-up services out there).
I think the attitude is a bit strong to potential contributors, even though I can see the reason why it's bad for an established project to consider any migration. It's a personal preference. If you are the main author and you don't want to move, you don't have to move. I don't have time to manage a full system. Heck, I don't have any open source code that is popular enough warrant me to think about running a mailing list or a buildbot farm. Github is perfect for me for just tracking and sharing code with the world.