Many players found it strange that the expansion was available on disc in the Xbox version, but not in the PlayStation version. John Linneman, from the Digital Foundry channel, is the one who provided an explanation for what happened.
According to the journalist, the reason would be that Sony does not allow the DLC to be included on the disc separately, meaning it is necessary to create a new build of the game with the DLC already included. The reason CD Projekt didn't opt for this would be that it would cost time and money to have the new build resend for Sony to approve.
Ahhh, I know what's happening. This is a problem with the Sony backend. They don't let you include DLC on disc like that without basically creating an entirely new SKU. Other developers had the same problem.
They would have to recompile a version of the game with the DLC built in, which means resubmitting it to Sony. They probably didn't end up spending the time and money involved. :-/
According to John, other games have already suffered from the same problem on the PS5, such as Final Fantasy VII Remake. As for the lack of DLC in the PC disc version, the reason would simply be that there would be no financial incentive for the studio, as the Blu-Ray player is not popular on the platform.