It's in the F.A.Q:
"Why is the PC version of Shenmue III not offered on this website?
At the moment, we are discussing the PC version with the related parties."
I am guessing that the pc version may have been a kickstarter exclusive in the license due to sony's participation. Pretty crappy if the case.
Edit: saw a twitter that there apparently was an issue with offering the pc version in further crowdfunding campaigns. Kind of weird.
https://twitter.com/CedricBiscay/status/644515841841598464This comment was edited on Sep 17, 2015, 20:03.