Read this: Buddhism may have spread only slowly in India until the time of the Mauryan emperor Ashoka, who was a public supporter of the religion. The support of Aśoka and his descendants led to the construction of more stūpas (Buddhist religious memorials) and to efforts to spread Buddhism throughout the enlarged Maurya empire and even into neighboring lands—particularly to the Iranian-speaking regions of Afghanistan and Central Asia, beyond the Mauryas' northwest border, and to the island of Sri Lanka south of India. These two missions, in opposite directions, would ultimately lead, in the first case to the spread of Buddhism into China, and in the second case, to the emergence of Theravāda Buddhism and its spread from Sri Lanka to the coastal lands of Southeast Asia.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Buddhism may have spread quickly because of what Mauryan emperor?
Ashoka