The programming community became globally dependent on a single commercial company. Using GitHub would mean contributing to that dependence and monopolization even more.
It's not just about moving data from one provider to another. GitHub is not simply a hosting service, it's a large centralized social network, and the cost of switching from it includes reduced project visibility and disconnection from the community.
These may be issues for smaller projects, but I don't feel that either visibility or community is a problem for a project like Python. They have the luxury of going wherever they want and people will follow.
