Because the Purāṇas came later, the question looks better if rephrased as: Why is Purāṇic Indra so different from Vedic Indra?
The late P. L. Bhargava addressed this very issue in his paper The Origin and Development of Purāṇas and Their Relation With Vedic Literature. As he says, the authors of various Purāṇas are responsible for bringing disrepute to Vedic gods like Indra, Varuṇa, etc.
The religion of the Ṛgveda is very simple and there is hardly any room
for mythology in it. The later Vedas have also very little of
mythology. It is the Brāhmaṇa literature with which real mythology
starts. The mythology of the Brāhmaṇas is, however, mostly a natural
development of the beliefs recorded in the Vedas, though at places it
violates the spirit of the Vedic hymns. Later works like the
Bṛhaddevatā are much nearer the Purāṇas. As a matter of fact the
Purāṇas being mostly sectarian works, their main aim was to extol and
elevate their own particular god at the expense of other gods. The
Vaiṣṇava Purāṇas were particularly aggressive in this respect. They
naturally wanted the exclusive worship of Viṣṇu and since some of the
other Vedic gods, particularly Indra, stood in the way of the
ascendancy of Viṣṇu, they distorted the Vedic account of this god and
even fabricated new stories to bring Indra into disrepute. The truth
is that Indra was already a very popular god when the ancestors of the
Indians and Iranians were still one people. For this reason
Zoroastrianism in Persia and Vaiṣṇavism in India had to face great
difficulty in persuading the people not to worship Indra as a
competitor of Ahura Mazda and Viṣṇu respectively. The Zoroastrian
Avesta therefore converted Indra into a demon and the Vaiṣṇava Purāṇas
did everything short of turning him into a demon in order to bring him
into disrepute. Let us now see how the Purāṇas distorted the Vedic
image of Indra.
One of the chief epithets of Indra in the Ṛgveda is Vṛtrahan, which he
earned as a result of his victory over Vṛtra, the demon of drought.
This is the greatest exploit of Indra in the Ṛgveda. This demon is
pictured as a dragon encompassing the waters and by killing him Indra
releases water for the mankind and thus acts as the saviour of
humanity. In this fight the Maruts are his regular allies but Agni,
Soma and Viṣṇu also often assist him. Now let us turn to the Purāṇas.
The Bhāgavata Purāṇa has converted the dragon Vṛtra into a Brahman by
killing whom Indra incurred the sin of Brahmanicide (SB VI.12,
VI.13). This transformation is in itself amazing but when it is
added that Vṛtra was a devotee of Viṣṇu, one simply feels astounded
for in the Ṛgveda Viṣṇu helps Indra in the slaying of Vṛtra.
Another epithet of Indra in the Ṛgveda showing his great power is
Śatakratu which means one having a hundred powers. This epithet was
interpreted by the Purāṇic authors as meaning one who performs a
hundred sacrifices and so in consonance with this meaning it was
presumed that one who performs a hundred sacrifices becomes Indra.
Hence Indra has been shown as being constantly afraid of kings who
intend to perform a hundred sacrifices and trying to foil their
intention. One such example is that of Pṛthu. Another is that of
Sagara. The horses of both were stolen by Indra according to the
Bhāgavata Purāṇa (IV.19.10-11; IX.8.8). What a travesty!
From a lover of sacrifices in the Ṛgveda Indra has been transformed
into one who dreads the sacrifices in the Purāṇas.
Yet another epithet of Indra in the Vedas is Ahalyāyai jara which only
means favourer of the unploughed land. The personification of Ahalyā
began in the Brāhmaṇas but the Purāṇas fabricated a most unseemly
story wherein Ahalyā has been depicted as the wife of a ṛṣi named
Gautama who pronounces a very ugly curse on Indra for seducing her
(Brahma 87; Padma V.51). Thus the ṛṣis who used to worship Indra in
the Vedas have been given the role of cursing him in the Purāṇas.
In the Vaiṣṇava Purāṇas which give the story of Kṛṣṇa, Indra has been
several times depicted in an unfavourable light in order to prove the
superiority of Kṛṣṇa. A notable example is the incident of Kṛṣṇa's
substituting the worship of the Govardhana hill for that of Indra and
Indra's subsequent wrath and humiliation (VP V.10-V.12; Brahma 187-88;
SB X.24.7). Another example is provided by Kṛṣṇa's invasion of Indra's
heaven for carrying away the Pārijāta tree in order to satisfy the
whim of his wife Satyabhāmā and for bringing about Indra's complete
debacle (VP V.30; Brahma 203; SB X.59.39-40).
Another feature of the Purāṇic Indra is his fear of the demons for
which he constantly seeks the aid of human kings. Sometimes even human
kings are represented as subduing him (see the story of Raji and his sons, Vāyu 92.75-90; Brahmāṇḍa
III.67.80-96; Matsya 24.35-43; VP IV.9; SB IX.17.12-15). He is also
afraid of ascetics who by their austerities can capture heaven and so
whenever any ascetic performs severe austerities Indra sends some
Apsaras to distract his mind (see the story of Kaṇḍu in Brahma 178). Thus in the hands of the
Purāṇic authors the Vedic Indra has been transformed beyond
What applies to Indra applies in a lesser degree to the other great
Vedic god Varuṇa because of his much less popularity. This omniscient,
omnipresent and omnipotent god of the Vedas has become a god of pools
and puddles in the Purāṇas. One example from the Bhāgavata Purāṇa (SB
X.28) will suffice to show how completely he has been shorn of his
greatness by the Purāṇas. Once when Nanda, the fosterfather of Kṛṣṇa,
went to bathe in the Yamuna a servant of Varuṇa made him a captive and
took him to Varuṇa. When Kṛṣṇa came to know of this he went to Varuṇa
who, while beseeching him to take back his father, pacified him by
humble prayers and apologies.
Besides the denigrating of Vedic gods, the Purāṇic religion has many
other features such as the glorification of temples, the consecration
of images and the appeasement of planets which cannot be called the
amplification of what we find in the Vedas. Thus the claim of the
Purāṇas that they reinforce the Vedas cannot stand the test of
scrutiny at least in the sphere of religion.
That does not, however, in the least mean that the Purāṇas are not
valuable documents. In fact, we should be grateful to the authors of
the Purāṇas for preserving for us, even in a distorted form, a record
of the great deeds of our ancestors from the earliest times to the
fourth century A. D. Much of our history would have been a void if the
Purāṇas had not come down to us.