I'm going to guess food would be the top reason, followed by lack of predation. So many reef keepers would be thrilled to have that "problem" that listing your tank conditions/inhabitants might help provide a template. How cool!
It was cool until I realized I had no choice but make a dirty tank to provide enough algae! They start off too small to give them away. They grow really fast and they are now a size about a pinky nail. I guess I have to get rid of one which bums me out.