Wikipedia states that "over a quarter of the 1,208 hymns of the Rigveda mention Indra, making him the most referred to deity than any other."
But in the post-vedic texts his importance is greatly diminished.
What are the reasons for his loss of status?
In the wikipedia link for post-vedic texts section, it states:
In post-Vedic texts, Indra is depicted as an intoxicated hedonistic god, his importance declines, and he evolves into a minor deity in comparison to others in the Hindu pantheon, such as Shiva, Vishnu, or Devi.
While I agree with the second part of Indra seeming like a minor deity in comparison to others, his exact depiction is still varied in different accounts/literature and not always as an "intoxicated hedonistic god". The reason he is referred repeatedly in Rigveda is likely because he is referred to as the king of heaven:
Of the Vedas I am the Sāma Veda; of the demigods I am Indra, the king of heaven; of the senses I am the mind; and in living beings I am the living force [consciousness]. (Bhagavad Gita 10.22)
With the heavenly realm being the residence of all the demigods, Indra is also understood to be the king of demigods. The Vedas have many sections that encourage worshiping various demigods, and the king of all demigods thus understandably gets a large chunk of the attention. His status, however, is still subordinate to the holy trinity of Brahma, Vishnu and Shiva, as well as Durga (or Devi).
(This answer is based largely on simple logic than a lot of references. If those are needed, though, please leave a comment.)
