John Calvin Commentary Numbers 35:28

"because he should have remained in his city of refuge until the death of the high priest: but after the death of the high priest the manslayer shall return into the land of his possession." — Numbers 35:28 (ASV)

Because he should have remained in the city of his refuge. The period of banishment is prescribed, until the death of the high priest, because it would have been anything but humane that all hopes of restoration should have been cut off from the unhappy exile; and, when a new priest succeeded to reconcile the people to God, this renewal of grace was to propitiate all offenses. Therefore it was not unreasonable that God should entirely restore those who were only punished for inadvertency.