In the continuous torque zone, the torque is limited because the current is limited. And the current is limited because it gives rise to power dissipation in the wingdings. This is due to winding resistance. The power dissipation is I^2 * R. In many types of motors, including induction and BLDC motors, torque and current are proportional.
Before I go on, I need to explain something. For the most part, induction motors try to rotate at a speed proportional to the supplied AC frequency (minus a little bit of slippage). BLDC motors are synchronous to commutation frequency. For both, along the constant torque line, from low speed to rated speed, the built-in assumption in the graph you have shown is that the voltage and frequency are ramped up together. Said another way, on the constant torque line, V/f is held constant.
It is further assumed that from the rated speed to the maximum speed, the voltage is held constant at the rated voltage, while the frequency is ramped upward to increase motor speed. Since the motor is an inductive load, the increasing frequency at fixed voltage results in decreased current, and thus decreased torque.
Hopefully that explains everything.