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

An important social context for the rise of Buddhism had to do with developments within Hinduism. The rigid caste structure endorsed by Hinduism in the fifth century BCE had become distasteful to many who disliked the opulent lifestyles and corruption of the priestly Brahman caste. So when Siddhartha Gautama (himself a member of the priestly Kshatriya caste) became the Buddha, the Enlightened One, his message of simplicity and rejection of earthly pursuits resonated with people. Its rejection of caste especially appealed to people who occupied lower stations in society.
Another reason that Buddhism spread and flourished was that it enjoyed the protection of powerful political figures, including kings and emperors. Perhaps the most important of these men was Ashoka, the ruler of the Mauryan Dynasty who may have converted to Buddhism in the third century BCE. Under his friendly leadership, Buddhism spread throughout much of the northern reaches of the Indian subcontinent and into modern Afghanistan. Buddhism, like other religions, also spread along the trade routes that connected peoples throughout Asia. Known later as the "Silk Road," this was an important network for cultural exchange, including religion. In was along these trade routes that Buddhism reached China, the Korean peninsula, and Southeast Asia within five centuries of the Buddha's death.
http://www.silkroadfoundation.org/artl/buddhism.shtml

https://www.nationalgeographic.com/culture/people/reference/ashoka/

https://www.pbs.org/edens/thailand/buddhism.htm