In the universe that Rick Riordan created he includes Greek gods, Egyptian gods, Viking gods, and Roman gods. As we find out in The Heroes of Olympus series, the Roman gods are only varied personalities/manifestations of the Greek gods. After Rome had fallen the gods moved to the most influentially powerful place on Earth at the time (Germany, England etc...). We also see that the locations appear to affect the Greek manifestations of the gods.
Why don't the locations affect the personalities/manifestations of the Roman gods?