Greg’s answer was always that they didn’t expect any resistance on Voya Nui and were unprepared to fight anybody, but the real answer, obviously, is he needed them out of them out of the way so he could focus on the Toa Inika. The Piraka also stole their masks, so by the time they were ready to fight, they were weakened, and finally beaten by Brutaka, who was somewhat pumped up on Antidermis at the time. Also Tahu is an idiot