Why?
The ancient Romaoi gods were really the Greek gods under new names (Heracles in Greek became Hercules, Jupiter took on aspects of Zeus), because the Romaoi religion prior to 200 BC was primitive, the entire Italic one was a primitive Aryanic religion with deavas/diviius (house spirits -- that is the origin of the term divine) or genii of the house (origin of the term genius)
The Greek original pantheon (the Cretan and Mycenean) was pure Indo-European, but later took on aspects of Semitic religions (hence Europa etc.)
class="b2"> One interesting correlationship is between the Aryanic gods from Dyaus Pita (Sky Father) to Jupiter and Zeus and Deus
also the storm god, Indra = Thor.
And, finally the two "families" of gods. In the Indic religions these are Devas and Asuras.
You see initially in the Rig Veda how some of the gods are referred to as Asuras -- like Varuna (god of wind) and Agni (god of fire).
By the time of Buddha, the Asuras were relegated to a lower family of supernatural beings, not to be worshipped but feared (an aside -- one of the now-famous Hindus gods, Ganesha/Ganapati, the elephant-headed god started out around the time of Christ as an asura or a spirit of malevolence like the rogue elephants who destroyed houses, but now he's the most popular Hindu god -- check Ganeshutsav).
Then, Christianity came and highly influenced Hinduism, from the Hindus taking the concept of a Trinity (only completely accepted around 900 AD) to their relegation of Asuras as demons.
In contrast, the opposite happened in the Irani Aryanic group where the Asura got prominence. Now, between Indic and Iranic languages there is a change of the sound S to H (so the people of Sindh (Indus valley) became Hindus) and the Irani gods were the Ahuras, with the Daeves being relegated to household gods, gods of the fireplace. These daeves were the same as the genius fireplace gods in Greek and Roman myths.
Zoroaster, around 1800-1700 BC was the prophet of his religion Zoroastrianism -- he took ONE God, Ahura Mazda and raised him as THE God of light. He consider there to be an equal god of darkness, Aingra Mainyu who represented evil. The Zoroastrian belief is that mankind should fight for Ahura Mazda against Aingra Mainyu. They should do this based on the Zoroastrian motif of "Good thoughts, good words, good deeds". Later sects of Zoroastrianism even went so far as to say that some humans were made by Ahura Mazda (Ormazd in some English poems) and others by Aingra Mainyu hence the idea that Ahura Mazda only loved some. They also had the concept of a Messiah-type figure, angels etc. (though their angels are bulls with the heads of men and wings!). It is fascinating on this Zoroastrian influence on Judaism post the exile.
Anyway, I digress -- the other branches of the Aryan family were the Greek-Romans who reduced it to one family of gods. The Romanic religion was far more primitive than the Hindu/Irani and more primitive than the GReek, hence whole-scale borrowing from the Greeks, who in turn were influenced by Semitic religions of the Canaanites/Pheonicians.
The Nordics/Germanics held the primitive Aryanic religion retaining BOTH families of gods: Aesir and Vanir. they would tie for primitiveness of their religion with the Celts.
But the most primitive indo-european religion is what we see in the pre-Christian Baltics and Slavics -- essentially worshipping of a Storm god (Perun) and sky
