Why did the romans adopt the Greek gods instead of coming up with their own?

To answer this question we have to think about the characters of the Greek and Roman civilizations. The Greeks were a culturally prolific civilization that carried out a process called Hellenization. It consisted in transferring his belief system to the people who were nearby. In this case, we can not think in territorial terms but rather in cognitive terms. The Greek culture was absorbed because it was valid in itself. When the Romans invaded Greece, a warlike people accustomed to surviving, they knew how to preserve the features of the Greek culture that worked to maintain peace in their "colonies".

Then, although the Romans conserved the pantheon of gods of Greek origin, they re-baptized them, attributing to them relations with their own history, the history of Rome. Hence arises the well-known myth of the founding of Rome, in the hands of Romulus (and his brother Remus) who were direct descendants of Mars (god of war) who found his Greek equivalent in the god Ares.