Here's why:

* Speed is the rate at which an object moves, regardless of direction. It's a scalar quantity.

* Velocity is the rate at which an object moves in a specific direction. It's a vector quantity.

Similarly:

* Distance is the total length of the path traveled by an object. It's a scalar quantity.

* Displacement is the overall change in position of an object, measured in a straight line from the starting point to the ending point. It's a vector quantity.

In short: Speed and distance are concerned with the magnitude of motion, while velocity and displacement consider both magnitude and direction.