Because he saw in Dharmma a way to cause an organization of social norms.
Explanation:
Dharmma was not a religion, but it used Buddhist concepts and a mixture of different religions present in the kingdom to establish a series of social norms and behavior that should be followed by the population. With the creation of Dharma, Ashoka managed to ensure that these norms were followed by everyone in his reign, regardless of the beliefs they followed, even managing to make possible visitors, or foreigners, succeed in following them without hurting their religious concepts.
