Various Missing Items

Hello, I am designing a homebrew game. I have bought Volo's Guide, Mordenkainen's tome, the Monster Manual, Curse of Strahd, and Tomb of Annihilation. I've probably bought more, but I can't remember of the top of my head. I am just wondering, why am I unable to, in a tomb of annihilation game, access the Haunted one background from Curse of Strahd, or in the new homebrew game, I have to add things like the Flying Monkey, which is in Tomb of Annihilation?

This was recently brought up in another thread (or maybe the last Round Table?). IIRC, many of the older module conversions had Compendium elements that were not incorporated into the larger mature Compendium format. I believe that the intent is to eventually add these elements, but I don't know if there is a time table on this. So that's my half-baked, half-remembered answer. :/