Sliding friction is less than static friction because the surface is smooth and the pressure applied to the surface is less due to less contact to the surface
In static friction the body is stationary, not moving. For eg. a box is lying on the floor.That means that no other force is acting against the friction. so the static friction has the complete hold, grip over the object and thus is the greatest type.

In sliding friction an object is moving over another object or surface. Like a person pushing an already moving box to keep it moving with the same speed. In this situation the friction is experiencing an opposition by some other force, here, muscular force. 
So that is why sliding friction is less than static one.