Question in regards to why I can use my modules compendiums in non module based games

This is a question in regards to why one cant access something provided in a module in a game that isnt a module For instance, I am running a homebrew game and there is stuff in princes of the apocalypse, such as monsters and enemies I would like to utilize for the game, but I cant access them unless I am running a Princes of the Apocalypse module based game Is there a reason for this restriction?

I think this is more of an account level restriction than anything else, due to being unable to freely export entries from your character vault into a game that isn't based on a module that includes vault access.  Plus subscription:  Move monsters/enemies from the module based game to the custom game through the character vault. Pro subscription: Move monsters/enemies, handouts, or maps from the module based game to a custom one with the transmogrifier.

Many of the modules were created well before 5e compendium integration existed. If I'm remembering correctly, Ghosts of Saltmarsh was the first module to offer compendium integration for it's contents. Roll20 hasn't gone through and retro-fitted older modules into the new compendium system, although they have mentioned in some of their developer streams it is something they would like to do.