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Because the early vedic religion lost its simplicity and people began to belive in spells and magic.people lost faith in such a religion and yearned for a simple  religion. at that time buddhism and jainism got up following love ,pity and kindness.they preached their religion in parskrit (common language used by people)thus buddism and jainism became popular
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As soon as vedic religion lost its glory and ppl believing in superstitions;these 2 religions become popular

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