This is how I understand it (and feel free to correct me anyone who's an expert at this): The thing with brushless motors is, you have to know where the rotor is in order to give it max power. To figure out where the rotor is, you need an ESC. This is where the low startup torque situation comes in. In order to find the position of the rotor on a sensorless motor, brushless ESCs use back-EMF that's generated when the motor is spinning. But when the motor's at a standstill or moving very slowly, there's barely any back-EMF to measure, so the ESC has to do a lot of 'guesstimating'. So until the motor's spinning fast enough, you can't give it max power, hence the low startup torque.