Why is the Stranger Things Chapter PS4 to PS5 upgrade disabled?

Due to licensing agreements and local regulations requiring a paid upgrade, we are not able to offer the PS5 upgrade of the Stranger Things Chapter to PS4 users in Germany and Japan.

Players who own the PS4 Stranger Things Chapter will still be able to play the content on their PS5, using the account it was purchased under. However, players will not be able to ‘import’ ownership on a PS5 console through upgrading, or family-share this content with other users on the same device.