The best explanation of the events that led to the spread of Buddhism during Asoka's reign is that "Asoka adopted Buddhism" and "Asoka became a just and compassionate ruler."
Asoka, initially a warrior king, converted to Buddhism after witnessing the suffering caused by his campaigns, particularly during the bloody conquest of Kalinga. His conversion to Buddhism led him to promote its teachings of compassion, non-violence, and morality across his empire, which significantly contributed to the spread of Buddhism both in India and beyond.
Thus, the key points are Asoka's adoption of Buddhism and his commitment to being a compassionate ruler.