This is meant to provide an archive for the code. A major attraction of using GitHub is to allow easy pull requests and issues. If you're not going to be using those features, it's mostly useless when you already have your own infrastructure for hosting data. Lastly, if you're using a version control system other than Git, GitHub is not going to work well.
You can set repos to be archived, i.e. read-only, on Github these days.
Besides, I find it a whole lot easier to just casually browse through code on Github rather than download 129 megs of eudora_w_source.zip, extract it somewhere and rifle through it...
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.
