[maxlybbert]

This isn’t the first project the Computer History Museum has hosted. At this point, I’m sure the thinking is simply that they self-hosted the previous source code drops.

I can imagine additional reasons (e.g., no desire to tie the museum to a third party, even if that party looks like it will be around for a while; who knows what policies Github might adopt in the future or which groups of people they may manage to upset?). But to be honest, I doubt a lot of thought went into this decision, this time.