This is because the projectile is under the influence of single force; gravity. Gravity acts vertically downwards. So, if there is any change in velocity, it will be in vertical direction. Gravity does not affect the motion in horizontal direction. So, there is no change in the horizontal component of the velocity of the body.
This is because the projectile is under the influence of single force; gravity. Gravity acts vertically downwards. So, if there is any change in velocity, it will be in vertical direction. Gravity does not affect the motion in horizontal direction. So, there is no change in the horizontal component of the velocity of the body.