Why did "Indra" lose his importance since early vedic period?

Over a quarter of the 1,028 hymns of the Rigveda mention Indra, making him the most referred to deity than any other - Wikipedia.

According to Ashwa Sukta, Rig Veda 1.163.2.1 Indra is the first to mount the horse. This means that Indra is the first god for converting an idea into an action. This is the reason why there are more verses about Indra in Rig Veda than about any other god.

However, in the Puranic period Indra is cursed by Gautama muni for having an affair with his wife Ahalya.

Indra is again humiliated by Krishna, who lifts Govardhan hill to save people from downpour caused by Indra.

Why did the most important god of Rig Veda became so unimportant in the Puranic period.

Rig Vedic verses are about how to convert an idea into an action using the seven stage muscle tone based thinking. This ensures that the idea becomes compatible with all other ideas / actions of life. In this apporach Indra was the first god.

However, though this is the ideal approach to life, most people found that the idea stagnated at one of the seven stages and very rarely got converted into an action, even if the Hindu had years of practice of Vedic Meditation.

In the Puranic period the fourth, or the central god, Vishnu became the default god, instead of the first god Indra.

Thus, instead of using seven gods in series, a Hindu could use just one god. However, this made it extremely difficult to reach the most peripheral gods Indra and Yama from the central default stage and have control. Thus, Indra was made the god of Heaven and Yama, god of Hell.

Thus, Indra, the default and the most popular Rig Vedic god,

became practically a non entity in the Puranic period.

What is the relationship between Harappan and Vedic civilization?

 For decades, textbooks taught that invading Aryans violently destroyed the Harappan civilization. In reality, they arrived to find only decaying brick ruins.

The relationship between the two is defined by a wide chronological gap and a slow cultural synthesis. The Harappan, or Indus Valley, civilization peaked between 2600 and 1900 BCE. They were mercantile city-builders who constructed mathematically planned urban centers with fired-brick architecture, complex drainage systems, and an undeciphered written script. By 1900 BCE, shifting monsoons and drying rivers forced the Harappans to abandon their great cities and disperse into smaller rural settlements to the east and south.

When Indo-Aryan speakers—the authors of the Vedic civilization—began migrating into the northwestern subcontinent from Central Asia around 1500 BCE, they did not encounter bustling urban fortresses. They walked into a landscape of long-abandoned settlements like Mohenjo-daro.

Excavated ruins of Mohenjo-daro in modern-day Pakistan. By the time Vedic culture emerged in the region, the advanced cities of the Harappan civilization had already been abandoned for centuries. Photo by Saqib Qayyum is licensed under CC BY-SA 3.0.

The two cultures could not have been more different in their prime. The Harappans were highly organized urban merchants with little evidence of large-scale warfare. The early Vedic people were semi-nomadic pastoralists whose wealth was measured in cattle and horses. They spoke early Sanskrit and transmitted their foundational religious texts, the Vedas, entirely through oral poetry rather than writing.

Despite the centuries separating their respective peaks, the two civilizations are deeply connected. The Harappan people did not vanish when their cities crumbled; they simply became village farmers. As the Vedic culture expanded across the subcontinent over the next millennium, it absorbed the remnants of Harappan society.

This blending process laid the foundation for classical Indian civilization. While the Vedic culture supplied the Sanskrit language and the philosophical framework of the Vedas, many scholars trace elements of later Hinduism—such as reverence for the pipal tree, ritual bathing, and early meditative postures—back to the silent, brick cities of the Indus Valley.