Why does only vertical acceleration change in projectiles?

 

Physics explains the movement of projectiles as a composition of movements, that is, there is a movement in the horizontal axis (Uniform rectilinear movement) and a movement in the vertical axis (Uniformly accelerated Movement).

A motion is rectilinear when an object describes a straight path at constant speed, since its acceleration is zero.

So in projectile motion there is only acceleration on the vertical axis.