Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized.
* Examples: John, New York, Mount Everest.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Examples: man, city, mountain.
"College" is a common noun because it refers to a general type of educational institution.
However, when "college" is part of the specific name of a college, it becomes a proper noun and should be capitalized.
* Example: Harvard College is a proper noun because it's the specific name of an institution.