Question

why is sound wave is called a longitudinal wave

Answer 1

Sound wave is typically a pressure wave and we all know that pressure waves are longitudinal waves.
Pressure waves moves in regions of compression and relaxations instead of creat and trough.

Answer 2

$\large\mathfrak\purple{→}$ The vibration of the medium that travels parallel to the direction of the wave or along in the direction of the wave, is called a longitudinal wave. The direction of particles of the medium vibrates parallel to the direction of the propagation of disturbance. Therefore, a sound wave is called a longitudinal wave.