Final answer:
Theravada Buddhism gained popularity among Southeast Asian political leaders as it was a tool for legitimization and power projection, influenced by the example of the Indian King Ashoka and the adaptability of Buddhist principles to local contexts without enforcing a rigid caste system.
Step-by-step explanation:
The popularity of Theravada Buddhism with political leaders in Southeast Asia can largely be attributed to its utility in establishing and legitimizing power. Early Southeast Asian trading states, through their interactions with India, adopted many aspects of Indian culture, including Buddhism. Leaders like the famed King Ashoka of India used Buddhism to assert their legitimacy and display their capacity to patronize large-scale religious projects, which embodied both spiritual merit and control over resources. This example set by Ashoka resonated with Southeast Asian rulers, who recognized the potential of Buddhism, specifically Theravada Buddhism, to foster legitimacy and reinforce social hierarchies without the rigid caste system emphasized by Hinduism. Thus, political leaders eagerly embraced Theravada Buddhism as it provided them with an avenue to project their power, cultivate prestige, and maintain social order in their realms.