Here's why:

* Gravity: This force pulls the projectile downwards towards the center of the Earth. It's the force that causes the projectile to follow a curved trajectory and eventually return to the ground.

* Air Resistance (Drag): This force opposes the motion of the projectile through the air. It depends on factors like the projectile's shape, speed, and the density of the air. Air resistance can significantly affect the projectile's path, especially at high speeds.

* Other Forces (Negligible): While theoretically possible, other forces like wind or the Magnus effect (caused by spin) are typically considered minor compared to gravity and air resistance in most projectile scenarios.

Important Note: In many introductory physics problems, air resistance is often ignored to simplify calculations. However, it's crucial to understand that it plays a significant role in real-world projectile motion.