Śvētāmbara Jainism isn't actually that intense in its demands. It's very liberal in its demands of gender (nearly 75% of the Jain monastics are Jain nuns), allows a far greater importance on the deification of specific Tīrthaṅkaras. The issue is the reforms of the Śvētāmbara came late, c. 4-5th century BCE.
The problem with the spread of Jainism is that it was a philosophy of the Pre-Aryan upperclass that was reflected as the systems of thought came to the for once more. So not only were they disregarded by the Vedic priesthood on a ritual level but the commoners as well. By the time it was a faith that was ready for the masses, it was already in competition with the far more structured and unitary Buddhism. Also Jainism disregards the devas of folk cults and the Vedic pantheon as mediocre and worse than humans whereas Buddhism presents them as beings that are on a higher existential plane, unenlightened, but with an aura of holiness about them.
So earlier and various reforms in Jainism are needed for it to flourish, as well as raise in popularity.