[Dr Kain]

Simple, they need the show to last a full year. If they went the full armada out immediately, the show would be done in 3 episodes. :-p

Seriously though, I've never understood that. You would think that after the third failed monster they would just say screw it and go all out. In some series, like GoGoV or KakuRanger, it makes sense. In others, like Gokaiger or even DaiRanger, it does not.