Comment by compiler-guy

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.