>>52Because of the allusion to living under a bridge and demanding a toll (users' time) to pass (continue actually discussing). Once the toll is paid, the troll does not die, but instead just goes right back under his bridge, waiting another opportunity.
I have my suspicions that the term actually originated from the regenerative capabilities of trolls in certain archetypal games, but the canonical reason I've always been told is the bridge analogy. (Plus, flames don't actually kill trolls, they make them stronger.)