How was the spread of the Buddha’s more radical teachings a gradual process in India?

The Buddha's teachings spread mostly through the merchant class. The merchant class in India was unsympathetic to the Brahmin aristocracy. Brahmin faith contained a great deal of ritual and ceremony. Some rituals were caste-specific and could only be performed by Brahmin priests. Thus, the merchant class viewed the Brahmin faith as an expression of caste-based elitism.
The merchant class was mobile, wealthy, and open to religious reformation. Through their extensive travels, merchants brought the Buddha's teaching to major cities across northern and southern India. The cities of Khotan, Bukhara, and Merv were particularly recipient to Buddhist teachings. In these diverse and populous cities, Buddhist teachings evolved and merged with preexisting religious and cultural norms. The Buddha's teachings were simple and did not involve a lot of ritual. For this reason, the teachings were easy to export to peoples of different cultures and ethnicities.