The Encyclopedia Eorzea is actually pretty clear about this, the Sultansworn only have half of the force that they need, they're weakened enough that they've had to receive aid from the Brass Blades to secure the palace. So it isn't purely a case of an incompetence or corruption on the Sulatansworn's part, though those likely play a part, but that they have been systematically weakened by the Monetarists, so that they could have their muscle there to "protect" the Sultana.