In my personal opinion, just as someone mentioned before, Jazz Jackrabbit was a parody of games and films (and also turned out to be pretty good by itself), which doesn't mean it neccesarily needs to have a sequel. At least from the point of view of the creators, they made a game which is complete in all ways, they probably don't see many reasons to create a new Jazz game, it would only satisfy a small fanbase and it seems like the creators aren't interested in JJ so much. I find the release of the games on GOG as a satisfying step, people can finally buy these games easily, and that's pretty much all they seemingly wish to do with the JJ series.