As long as gods were entertained, there was need to propitiate them. Rituals became imperative. Reflection and introspection changed priorities. Comfort in life became secondary. Acquisition of knowledge became primary.
The thinkers, who initiated the Upanishadic thought, were not happy with ordinary knowledge. They argued that knowledge obtained from the Vedas was only lower knowledge (aparaa vidyaa), which is impermanent. What they sought was permanent knowledge (paraa vidyaa). Prolonged performance of yagas must have dented all hopes of seeking knowledge.
With gods, rituals also made an unceremonious exit. To make qualitative distinction the Upanishads were called Jnana Kanda in contrast to the Brahmanas, which were termed Karma Kanda. This distinction could be achieved because of internal critical attitude.