What were the social and cultural conditions that informed and set the stage for the birth and spread of Buddhism?

In India, Siddhartha Gautama was not the only religious person who was dissatisfied with the status quo. Wandering lay people, or ascetics, believed in living away from worldly superficiality and materialism. After he reached the enlightened state, becoming the Buddha, his teachings spread even further. After his death, others shared his beliefs in northern India, where some followers established monasteries, and then extended north into the Himalayas and south into the Subcontinent. The Buddhist movement gained considerable weight when King Ashoka, whose territory roughly equaled that of contemporary India, converted and made Buddhism the official state religion.
Ashoka’s grandfather, the emperor Chandragupta, had already turned away from Hinduism, effectively challenging the control of the ruling class, or Brahmans, who used divine mandate to justify their control. He ended his days as a Jain monk. Under Ashoka, religion was a key element of political control. He aimed to show his people a pious example of rulership. During his reign, not only did political control extend farther than previous monarchs had achieved, but patronage of Buddhism through written texts as well as monasteries expanded tremendously. By combining religious values with his diplomatic and economic endeavors, Ashoka promoted (although did not always achieve) harmony within his kingdom. The success of Buddhism in India paved the way for its expansion into other kingdoms and later into China.
https://www.britannica.com/topic/Buddhism/Historical-development