Question

why the vertical component of velocity in PROJECTILE motion not remains same?

Answer 1

coz gravitational acceleration acts verticallly

Answer 2

The vertical component remains the same because, the object accelerates towards earth, no matter what. This is gravity like he said. But because gravity is about 9.8 ms^-2 no matter where you are at any point of time, and because acceleration (positive or negative) changes your velocity by speeding you up or slowing you down, the velocity keeps changing. And that is why the vertical component of velocity keeps changing.

For the horizontal component, there is no force (like gravity) acting on it. This means there is no acceleration in the x-direction and hence the velocity remains the same.