reference- HC VERMA Q-45 Work and Energy

Why can't we apply the concept....restoring force,

kx=mg so that x=mg/k?

P.S.- I know  how to get the correct answer by applying conservation of energy.

Consider the situation shown in figure.

Initially the spring is unstretched when the system is released from rest. Assuming no friction in the pulley, find the maximum elongation on the spring.

Why can't we apply the concept....restoring force,

Because in that case.. It will represent the equilibrium position of spring.. But actually when system is released.. It will first shoot the equilibrium point and then comes at equilibrium position.. So by applying that.. U will get the elongation at equilibrium position and not the maximum elongation