FB has thrown a lot of random stuff over the wall that they use internally and thought might be useful to other people. I think Hack and Buck fit into this category. Facebook will maintain them forever, because they are core parts of internal infrastructure. Their value to Facebook is completely independent of wider industry adoption.

Whereas PyTorch is intended, from the ground up, to be a widely useful project, and the dev team weights open-source issues at least as much as internal ones.

(Full disclosure: I used to work at Facebook, including, briefly, on PyTorch)