Here's why:

* Gravity: The only force acting on the projectile in a vacuum is gravity. This force acts downwards, constantly accelerating the projectile vertically.

* Constant Horizontal Velocity: In the absence of air resistance, the projectile's horizontal velocity remains constant.

The combination of these two factors results in a parabolic path:

* Upward Trajectory: Initially, the projectile has an upward velocity component. As gravity acts on it, this upward velocity decreases until it momentarily becomes zero at the highest point of the trajectory.

* Downward Trajectory: After the highest point, the projectile starts falling downwards, accelerating due to gravity.

* Horizontal Motion: Meanwhile, the horizontal velocity remains constant, causing the projectile to move forward at a steady rate.

Therefore, the projectile traces a curved path, which is a parabola, as the combination of constant horizontal motion and constant vertical acceleration due to gravity creates this specific shape.