Why is synchronous motor more efficient than asynchronous motor?

The speed of synchronous motors is synchronized with the frequency of the power grid, without the need for sliding losses, making them more efficient in large power systems.
Asynchronous motors require friction and resistance losses between the sliding rotor and the row rotor, resulting in slightly lower efficiency.

Electric machines, in other words, are two magnetic fields, the winding (primary) magnetic field and the secondary magnetic field, interacting with each other, just like two magnets using attraction/repulsion to form motion.
Asynchronous motor, another professional name is induction motor, because its secondary magnetic field is induced, that is, a part of the energy from the primary winding needs to be transferred to the secondary, generating induced current or induced magnetic field, which will generate heat and other losses on the secondary.
In synchronous motors, both the primary and secondary magnetic fields are independently generated. Most synchronous motors use permanent magnet materials for the secondary, which means that their magnetic field is brought by the material and does not require additional energy input to generate.