Becuase they grow quicker than people think, then you end up with a 10 foot monster that you have to deal with. 95% of them end up at the door steps of local aquariums/zoos - who don't want them either. 4% of the remained get dumped into the ocean - which is highly illegal, messes with local environments and introduces zoonatic dieseases to the wild - which can wipe out wild populations.