It's just that...
Since those Divine Coins are being used as currency, their value could not remain so high indefinitely. And based on the situation at the time, those Divine Kings, in order to economize effort and to realize the intended value of the Divine Coins, did not expend much effort on the rule projection inside the Divine Coins.
As such, although shattering a Divine Coin back then could aid a Deity in cultivation, the lack of significant effort meant that even though the Coins had considerable effect, the duration of the rule projection was not long. Typically, the rule projection inside a common Divine Coin could only persist for a few days.
