Question

Why is sliding friction less than static friction?

Answer 1

Sliding friction is always less than static friction because two sliding objects find less time to get interlocked against each other irregularities of surfaces as a result of which they experience less friction.

Answer 2

the Sliding friction is always less than static friction because two sliding objects  take less time to get interlocked against each other’s irregularities of surfaces as a result of which they experience less friction.

for an example:- 
 the force required to overcome friction at the instant an object starts moving rest is a measure of static friction.on the other hand ,the force required to keep the object moving with the same speed is a measure of sliding friction . when the box starts sliding the contact point on its surface, do not get enough time to lock into the contacts points on the floor.so,the sliding friction is slightly smaller than the static friction.