why the power factor of a transformer is poor at no load?

The power factor of a transformer is poor at no load primarily due to the absence of a significant load current. At no load, the transformer primarily draws magnetizing current, which is mostly reactive. This reactive current is necessary to establish the magnetic field in the core but does not contribute to real power. Consequently, the power factor, which is the ratio of real power to apparent power, remains low under no-load conditions.