Can someone explain more about what happened here?
D&D 5th Edition (and some earlier editions) was released in a way that allowed third parties to create their own compatible content by referencing a document called the "SRD", licensed under the so-called Open Game License. This document contained the basic rules and content necessary to play D&D. If you wanted to include a zombie in your published adventure you could use the stat block from the SRD. The license also…
To expand on this about this, nothing around the existing v1.0a actually had field of use restrictions that would prevent its use in a VTT, and the accompanying contemporaneous FAQ indicated that using it for software was permissible. While VTTs as they exist today didn't exist when they wrote the license, character builders and video game adaptations did, so it's not like some VTTs are some totally unforeseen and therefore not plausibly covered.
So most VTTs are relying on the OGL where they just implement D&D and basic classes etc., the ones with deals are the ones that use additional non-OGL content which is why you can e.g. buy Curse of Strahd for roll20 or fantasy grounds despite Strahd not being OGL.