The reason for incompatibility is the DRM systemOriginally Posted by CaptainCanuck89 Go to original post
Steam and Uplay are DRM software as well.
Each Client is managing the game files
Thing is, when you get game on Steam witch will download the base game, and later you get Season pass on Uplay, the Uplay will download the game files.
Steam in this case is managing base game, but it has no access to Season pass witch is on Uplay.
Also Steam does not know that you got Season pass.
It has been like that since Assassins Creed 2, and as long there will be 2 DRMs, there will be no solution for that issue.