I haven't played State of Mind, but the developer is the same people who did Moment of Silence and Mystery of the Druids. If State of Mind was recommended to you, that is probably why.
Intended game sequels usually aren't made not because the developer lost interest or could not come up with more ideas, but rather because they did not have the money to make another game. A second game is often contingent upon making enough money from the first one; if that does not happen, the game gets canceled. Unfortunately this is not an uncommon occurrence as it's a tough market out there.
Most of my favorite adventure games were made in the 1990s, although I have played a few adventure games post-2000 that I also liked very much such as The Lost Crown.