Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: car, dog, city, river, book
* Proper noun: Ford Mustang, Spot, New York City, Mississippi River, "The Lord of the Rings"
Since "motorcycle" refers to a general category of vehicles, it is a common noun.