Question

why it is easy to maintain the motion rather than to start the motion

Answer 1

to start the motion we need acceleration ( which requires force) & if we are already in motion there is no need to acceleration much.
it is similar to the theorem of potential to kinetic energy.

Answer 2

because in start the motion we need a lot of potential energy to change in kinitec energy which help the object to comes in motion.
so it os more difficult rather than to maintain motion.