Question

why cant we hear ultrasonicwaves

Answer 1

The receptor nerves in our ears are sensitive to certain frequencies, and out brains process these vibrations into what we perceive as sound. "Sound" per se, does not exist, just vibrations. The tones that we 'hear' are the way our brains present this information to our conscious and unconscious awareneses. 

Answer 2

because20,000 Hz. Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz and beyond.  on only human brain can hear up to 20 heard to 20000 approximately  heard  thats y we cant hear it