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.