Part of the issue is that the Compendium and module conversions developed side-by-side. Originally, only specific books were converted, i.e. the Monster Manual. The tools were not in place to do this for modules, which are used to build games. Actually, they are essentially just copied pre-set games. As the systems matured, it became possible to tie elements from modules into compendium entries. Virtually every sizable module since this tech became available has had compendium access as part of its purchase. I believe I have heard (though this could be my faulty memory) that it is a commitment from Roll20 to eventually add compendium elements to older modules. When in doubt, check the Marketplace page. It should tell you if there is Compendium content. Finally, DnDBeyond has a lot of features that are not possible with Roll20, because they have a deeper partnership with WotC than any other developer, sharing marketing, for instance.