Source: TW
Sermon failure
Bauddhas learned the hard lesson that merely imparting an endless series of boring moral sermons doesn’t do it for most people. Maybe a few may change because of their innate Svabhāva/proclivity, due to which they are open to hearing criticisms of their conduct and acting on good advice. But most won’t. If it was the case that moral sermons will change people just by them hearing such pithy words, this world would have been substantially better than it is today, for there has been no lack of pithy moral discourses or their exponents.
Metaphysical layer
Hindus realized that beyond the body and mind network, there is a deep, subtle and imperceptible, metaphysical layer. Peel the external world (as if you could) and you would see a whole machinery of supernatural forces, including the Gods and other, lesser beings.
Therefore, starting from the Vaidika religion and throughout the Paurāņika and Āgamika phases of history, Hindu sages saw various rituals and ritual systems to make sense of the labyrinth of metaphysical entities (which are controlling us from within; some good, some very exalted, some not so good) and harness them for the good of all.
Mantras have different effects on the one who recites or hears them. When the deity of a temple comes out in a procession, showered with a sonic stream of mantras and presented with consecrated substances/offerings, and is seen by all, even deeply hedonistic people are gradually affected, with their beginning-less chain of karmas (which hardened their heart and made them unable to think beyond matter & utility) slowly eroding.
Reformation
Bauddhas and Jainas eventually realized that constantly harping on maitrībhāva (often translated as ‘loving kindness’ in English translations of Bauddha works) or using a fan to keep away ants or plucking one’s hair simply do not cut it.
They slowly adopted mūrtipūjā, complex mantra practice & even Agnikārya (fire rituals) and came up with grandiose metaphysics and ontologies with complex rituals based on those new insights. In short, they Hinduized and survived.
This is the Śramaņa story.
मानसतरङ्गिणीकृत् - टिप्पनी
I have long suspected that founders of those counter-religions themselves realized this during their lifetimes and thus sowed the seeds for H traditions coming back in through backdoor. Thus, we have new names of old archetypes from the parent religion.
Some might argue that these tales were retrofitted on to the founders as Hinduization was progressing through their traditions. That is true of some mythemes like in ashvaghoSha’s work or the much later kapphiNAbhyudaya written in elegant language. However, we believe the confrontations, such as that between urubilva jaTila kAshyapa and the tathAgata were original and integral to the development of their founders.