Laphicet's gift to the world wasn't merely to turn demons back into humans- it was specifically to give people a second chance. Another shot at becoming someone better than a creature filled with malevolence. The reason why Rokurou didn't turn back into a human is because for him, becoming a demon was that second chance. What little we know of Rokurou as a human depicts an asshole consumed by jealousy and anger who sold out his brother just to have an excuse to try to kill him. He doesn't want or need a second chance at being the sixth son of the Rangetsu family. He has his own creed to follow now.