It's basically because they have tons of superpowers
  1. Some can glow in the dark
  2. Some can clone themselves
  3. Some are immortal
  4. Some are smaller than a pin head while some are bigger than humans
  5. They don't have brains
  6. They have been really successful at shutting down nuclear reactors
  7. The box jellyfish is the most dangerous creature in the animal kingdom
  8. They're basically taking over the oceans
  9. They're one of the oldest complex organisms on earth. When jellyfish first started swimming 500-700 million years ago, the land on earth was lifeless. Not even plants.
    Suggested by @gwcoffey
  10. They are nature's lava lamps
    Suggested by @gilbaron