Unfortunately, it is because the game has that much depth that saves become incompatible. There were lots of changes to stats, skills, abilities, talents, etc, which would make importing old saves a challenge, and make tracking down bugs unnecessarily complicated.
You can disable auto-updating for the game in Steam (and if you let the update start and pause the download, you can leave it that way indefinitely and continue with the previous version). I don't think you can revert to a previous version, though.
