I believe the answer is: It caused the Roman empire to have a small and weak army.
By enforcing conscription, the majority of Roman soldiers were brought out to aid in the conquest to expand the Roman empire. When this happen, the defense for within the Capital become extremely weak and fragile. This make it easier for the invaders to attack and destroy the capital.
"Their conscription were a failure due to not enforcing to everyone. As
such there was not many people to choose from. It weaken Rome because
armies need manpower to hold the front since equipment is useless if
there is no person to use it. It also didn't enforce it on mostly on the
useless people like the rich who don't contribute very much to society
and the cooks and bakers who can't cook if all the farmers were
conscripted which would mean no food. This weaken Rome more than it
benefited her."
