As I said it has to do with the history not philosophy.
India entered the golden age of enlightenment around 400 BC.
This is the age when scholars started the creation of Upanishads. Upanishads basically took all the good of Vedas and discarded all practices which were no longer valid with the change in time. One of the key practice that was to be adopted was the concept of vegetarianism and discard the practice of animal sacrifices to please the gods.
But people have been so much tuned to these habits for ages meant that it was extremely difficult to implement these new concepts. Uneducated God fearing people were also worried about the wrath of the gods if they stopped the practice of animal sacrifices. So these new concepts remained limited to the educated classes of Brahmans (bureaucracy) and Vaishyas (businessmen) only.
This is when Buddha and Mahavira came in and basically took the atheistic position that there is no god. Since there is no god, people are no longer obligated to please the gods through animal sacrifices. Even with this position, it was difficult for people to adopt these new concepts. Only around 300 BC when Emperor Ashoka incorporated Buddhism as the religion of the state did Buddhism grow among the masses. Later Ashoka sent emissaries to SriLanka and rest of Asia to propagate these Buddhist principles. It is ironic that Ashoka was able to spread atheist concept of Buddhism but not the vegetarianism which was the primary reason why Buddhism adopted atheism in the first place.
A millennia later Adi Shankara toured across India to debate and convince the scholars that atheistic model of Buddhism has outlived its purpose and it was time to get back to the concept of believing in god. This is when India turned back to Hinduism from Buddhism.
Hinduism/Upanishads believes in the Big bang theory through the concept of Hiraṇyagarbha while Buddhism believes in the steady state universe to avoid the creation of the universe by god.
But Buddhism adopted atheism as means to implement vegetarianism and stopping animal sacrifices. Atheism was never the goal. This is the reason why it still retained the vedic/upanishad concepts of birth, death, rebirth, samsara and moksha/nirvana.
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