Here's why:

* Gravity: The only force acting on the projectile vertically is gravity. Gravity is a constant force, meaning it pulls the projectile downwards with the same acceleration (approximately 9.8 m/s² near the Earth's surface) throughout its flight.

* Constant Acceleration: Since the force of gravity is constant, the projectile's vertical velocity changes at a constant rate. This constant change in velocity is what defines uniformly accelerated motion.

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