"sorry if this has been asked oh so many times before, but, how come?"
Essentially, Rebellion no longer holds the rights to some content (notably the right to print/reprint Dan Dare), never had the rights to some stuff (creator-owned), and is involved in rights disputes over other properties (Zenith, potentially Luke Kirby, etc.) To that end, there will likely never be a 'complete' digital archive. Also, once you go back prior to 2000 AD's doing everything digitally, the quality's going to drop some anyway.