The reason is that the coastal states like California and NY tend to be liberal/democratic and the central states tend (strongly) toward conservatism.
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As far as why people on the coasts are more liberal and vote Democractic, and those in the interior are more conservative, here are a few variables that explain some of the reasons why:
Income - the richest people in America live on the coasts. The higher a person's income is, the more likely they are to be liberal. Even though Republicans do get alot of rich supporters as well.
Education - New England is where most of the best colleges are, and the Eastern population is far more likely to be educated than those in rural areas. Think of the populatiions of London, Paris etc. versus rural England. Same thing exactly.
Age - people in cities are young, people in the country are older. As a person ages, they don't want to live in a city anymore, and move to quieter parts. Older people are always more conservative (vote Republican) than younger people are (vote Democrat). Again, the rural parts are the interior, the cities are the coasts.
Diversity and multiculturalism - the rural areas of any country are always less culturally diverse than cities are. There are far fewer points of view and diversity of ideas in the interior of America than there are in the cities like New York or Los Angeles. Just like Londoners poke fun at "backwards" people in rural places. Exactly the same.
Those are a few, there are other reasons like history, heritage etc. For example, the Southern states like Georgia, Tennesse, Alabama etc. were slave states. The Northern states like Massachusetts (John Kerry) went to war to free the slaves. North and South really dislike each other.
Kerry is from a liberal Northeast, pro gay marriage multicultural state, Bush from a Southern ex slave state. Epitomizes the differences.