Because fully optimizing a kusari-gama build would be very feat intensive and MAD.
You'd need EWP, the 3 twf feats, combat expertise, improved trip, and probably weapon finesse. Improved disarm isn't worth it because it's a light weapon. So right there that's requiring 6 or 7 feats, 13+ int, and 19+ dex. And on top of that you'd need a secondary source of damage to make the twf worthwhile like sneak attack and multiple elemental weapon enchantments. Sure there's ways around that like swashbuckler, ranger, and wolf totem barbarian but it's a lot of work for not a significant payoff when you compare it to a typical spiked chain warrior.