3. They believe the IP may have potential that you won't exploit to the fullest.
In other words, they believe that they may be able to do more than you with it, if they ever get around to it.
If your potential market is tiny--and lets be honest, the market for an unpirated version of this is quite small,most people sufficiently interested have pirated it already--then keeping it out of the public in favor of some unknown potential later is a consideration.
I don't want to claim that NOLF is comparable to Zork, and without a doubt times have changed a lot since then, but it comes to my mind that it's a notable fact that even half forgotten and seemingly out-of-fashion IP can be much more valuable than anyone at any given time assumes.
Namely back in 1992, having the rights to Infocom's catalogue, which earlier had been put up for grabs for 25k, literally saved a then bankrupt Activision (!) when they rereleased it as a compilation of The Lost Treasures of Infocom, for millions of revenue in total. [1]
In addition to the problems and cost of establishing ownership that other people mentioned, I suppose this story was one hell of a lesson for any publisher not to forfeit any rights you may or may not be holding.
In other words, they believe that they may be able to do more than you with it, if they ever get around to it.
If your potential market is tiny--and lets be honest, the market for an unpirated version of this is quite small,most people sufficiently interested have pirated it already--then keeping it out of the public in favor of some unknown potential later is a consideration.