Thats just the rules of the game, that is a question for your gm to houserule if he wants.
A sai doesn't have a blade, so the manople is really more like a shortsword on a gauntlet. Thus I don't find it surprising that the manople falls into the same category as shortswords rather than the same category as a sai. About the closest things to the manople in the Close or Monk weapon groups are probably the punching dagger and the scizore (which is also in the Light Blades group besides being in the Close weapon group)
As to why the manople isn't in the Close weapons group too I can't be sure. I guess that the designers probably either felt it was too long or was just more like a sword on a gauntlet and less like a bladed gauntlet.
I wonder if a few levels of Titan Mauler would let you use a really big scizore which does 2d8 damage. It might be kind of amusing to flurry with that I guess.