Probably because Crate have improved/upgraded the GD engine so much that it wouldn't be that easy to do. It's now the GD game engine, not the TQ one. It could probably be done, but would mean a lot of work for a modder to do. Some things like the masteries and items, monsters, etc, have been brought over for GD mods like Grimarillion and Dawn of Masteries, but importing the entire world is another story methinks.
Also Crate Entertainment don't own the rights to TQ; that belongs to THQ Nordic, a different company entirely.