Because at that point I planned for us to go along the road and river, and I saw any threat that could move along the length of the river (you called it a tentacle) as a potential backstabber later. But that's neither here nor there.
Gromnir, you did express a little disappointment that the intricacies of your setting were not seized upon, and instead the campaign is trugging along like a "normal D&D campaign". Are the players satisfied with this, and are you? I mean, it's not like you don't have a right to derive enjoyment from the game just because you're a DM. You shouldn't drag the palyers the way you want to, but there's no reason not to continually offer creative hooks for the player to latch onto the psychic thing.