Here's why:

* Gravity: When an object falls, gravity pulls it down, causing it to accelerate.

* Air Resistance: As an object falls faster, it encounters more air resistance. Air resistance is a force that opposes motion.

* Terminal Velocity: Eventually, the force of air resistance becomes equal to the force of gravity. At this point, the net force on the object is zero, and it stops accelerating. This constant speed is called terminal velocity.