Oh for Pete's Sake, not another drama queen. Did Randy Pitchford make a friend? :P

To be fair, it's not like that general scenario hasn't happened plenty of times with digital re-releases of older games, either in general or on GOG specifically (i.e., later games in a series get released, some earlier entries are nowhere to be found).
The answer to your question, of course, is often--say it with me now--"rights issues".

Oh, that would have been nice.