Why did Pai-Mei teach Beatrix the Five-Point Palm Exploding Heart Technique?
Because he fell in love with her. In a realistic sense off course but he must have felt something for her. This love that she awoke in him summarises all of her qualities as a student and as a person as opposed to other students. For Pai-Mei to love someone that someone has to be special in many ways. So he granted her the wisdom.