For the very same reason the English language developed into different dialects in the US. People in New York call a car, a "Ka"; people in Texas might pronounce the word's "you all" as "Ya'll", people in Tennessee might say the same words as "Yu'n's."
Climate (the environment) probably has a major role to play in developing human cultures. California has a Mediterranean climate, and people in CA say "Car" and "You all." People in France, Italy, and California are more relaxed and take life as it comes; all three regions have a Mediterranean atmosphere (climate). All three countries (state) are heavy in the Arts, Entertainment (Hollywood is based in CA), Beaches, Wines, Orchards, Foods, Sports, and just enjoying life.
Conversely, all 3 nations (state) dislike war (and in the case of France & Italy, might not be to good at it...based upon history): Italy, was the ONLY major axis power to quit WW2 (they surrendered in 1943). The frozen cold countries of Japan and Germany went on to fight until 1945. The Mediterranean country of France, hasn't won a war since Napoleon! The Mediterranean state of California was headquarters of the Vietnam Anti-War movements (hippies).
Atmosphere, the climate can create cultures: Good climate creates friendly people; bad climate creates cold (hard) people.