There are mainly four reasons why this happened
- They lived in a period of great teachers (or close to their lives) who were thrown up because of the unrest in the society.
- The society in those times has not reached the level of rigidity that would fall upon it in the following centuries
- Brahminism was yet to completely exercise its hold over society. There was still considerable tussle for hegemony between the Brahmanas and Kshatriyas.
- Mauryan rulers were open and quite enlightened and open to new ideas.
1) Period of Great Teachers.
Consider this. We live in the period that follows two generations after Swami Vivekanada. The effect Vivekananda has on our generation is visible all around us. Now imagine a generation that lives during the time of, or follows, the Buddha. People would flock to such a towering personality like moths to a flame. The period from 6th century BC, like elsewhere in the world, was one of great intellectual activity in India. Buddha, Mahavira, Makkali Ghosala, Charavakas all these great people and ideas crowd the canvas of our history. It was difficult to not have been influenced by one of these ideas.
2) Less Rigid Society
Society in those times was not yet as rigid. The caste system was yet to entrench itself as the primary social theme. There were considerable sections which was not yet included or integrated into the societal fold. There were tribes in North West, uncharted territories of Peninsular India etc. This meant that society was mobile and hence had not yet been settled into a rigid framework.
3) Brahmanism was yet to establish itself
Brahmanism was yet to establish its supremacy. It was starting to lose its hold given the transition to agriculture during the later Vedic and Janapada period had started to erode the sacrificial religions role. Brahmanism would have to reinvent itself in the subsequent centuries by making a transition from sacrificial religion to Bhakthi or idol worship. There was also the constant tussle between the Kshatriyas and Brahmanas over their supremacy in the varna hierarchy. Buddhism for example places Kshatriyas over the Brahmanas. Needless to add, Buddha, Mahavira etc. were Kshatriyas.
4) Enlightened Rulers
Mauryan rulers were quite open to new ideas. Chandragupta thus being open to Jainism, Bindusara to the Ajivikas and Ashoka to Buddhism was a result of this enlightened policy.
Thus it was a combination of social, economic and political factors that contributed to the Mauryan rulers associating themselves with different sects of the period.