I think its because all the powers and tools the superheroes use are a bit too hard to make a plug-in for. I myself do not make plug-ins, but I'm sure it would be really hard to make a definitive superhero experience through plug-ins. Maybe a new update to minecraft will allow plug-ins to be easier? And yes, I know there are mods out there for superheroes, but modded servers can be very laggy.
