Here's why:
* Power Levels: Even after training with Kami, Piccolo's power level in Dragon Ball Z Kai is significantly lower than Freeza's. Freeza at his peak (final form) is many times stronger than Piccolo.
* Transformations: Freeza has multiple transformations, each dramatically increasing his power. Piccolo doesn't have comparable transformations.
* Techniques: While Piccolo has strong techniques like Special Beam Cannon, Freeza possesses a wider arsenal, including energy blasts, telekinesis, and his signature Death Beam.
* Experience: Freeza has been fighting and conquering for a long time, giving him more experience and cunning in battle.
It's important to note: While Piccolo is a powerful warrior, Freeza's overwhelming power and experience make him a formidable opponent that Piccolo wouldn't be able to defeat in Dragon Ball Z Kai.