Here's why:

* Gravity: The only force acting on the projectile is gravity. This force acts downwards, causing a constant acceleration of approximately 9.8 m/s².

* Constant Acceleration: Since the acceleration due to gravity is constant, the projectile's motion is uniformly accelerated.

* Upward and Downward: The projectile's velocity changes uniformly as it moves upwards (slowing down) and downwards (speeding up) due to the constant gravitational acceleration.

Important Note: While the vertical motion is uniformly accelerated, the horizontal motion of a projectile is uniform (constant velocity). This is because there's no horizontal force acting on it (ignoring air resistance).