Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples include:
* People: John, Mary, Queen Elizabeth
* Places: Paris, Mount Everest, California
* Things: Ford Mustang, Taj Mahal, The Great Wall of China
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples include:
* People: man, woman, child
* Places: city, country, mountain
* Things: car, building, tree
"Minivan" is a general term for a type of vehicle, making it a common noun.