There was nothing in the tenets of Jainism or of Buddhism that from a philosophical point of view need have caused a rupture with the Brahmans.
But the heresies, nevertheless, do not represent the priestly caste, so much as the caste most apt to rival and to disregard the claim of the Brahman, viz., the warrior-caste.

They were supported by kings, who gladly stood against priests.

To a great extent both Jainism and Buddhism owed their success (amid other rival heresies with no less claim to good protestantism) to the politics of the day.

The kings of the East were impatient of the Western church; they were pleased to throw it over.

The leaders in the 'reformation' were the younger sons of noble blood.