While some puranas describe Indra as a devotee of Lord Vishnu or other gods, the Vedas, in contrast depict Lord Indra as the most important gods. This is visible across the length and breadth of the Vedic literature.
However, in the puranas we find stories that Indra was defeated by Lord Krishna and others, Lord Indra was helpless and hence went and prayed to Lord Vishnu etc.
Why is that the importance of Vedic gods, especially Indra, Agni, etc. reduced after the advent of puranic worship?
Indra: About a quarter of the Rig Veda is addressed to Indra. There is hardly any hymn where Indra has not been celebrated. Even in the Soma Mandala, Indra has been called out as the Greatest Somapa (Drinker of Soma). He is the Satakratu (Performer of hundred yajnas), the Maghavan (Bestower of plenty), the Purandr (Destroyer of enemies’ forts). He is the Hero of the Heroes.
Vishnu: Vishnu is a minor God (less mainframe), linked to his 3 strides. Only 6 hymns of the Holy Rg Veda extol Lord Visnu. Vishnu has been variously called Urukrama (wide strides) Urugaya (Wide moving). In Rig Veda Vishnu has been several times associated with Indra. Vishnu accompanied the Great Indra in several battles against Dasas and A-suras, most significant being the annihilation of Vrtr.
Subsequent Vaishnav cult and Puranic literature reversed the roles, identifying Lord Vishnu with the sustainor and relegating Indra to a minor rain-God, who was vanquished by an Avatar - Lord Krshna.
What is then the reason that mainstream Vedic gods were replaced by others?
Who is responsible for this shift from Vedic to puranic worship?