Here's why:

* Vertical Motion: The projectile is moving upwards as it is launched, but gravity constantly acts on it, slowing it down.

* Maximum Height: At the maximum height, the projectile momentarily stops moving upwards. This is because gravity has completely counteracted its initial upward velocity.

* Instantaneous Velocity: Since the projectile is momentarily at rest at its highest point, its instantaneous velocity is 0.

Important Note: While the velocity is 0 at the maximum height, the projectile still has acceleration due to gravity, which is always acting downwards.