Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are the first word of a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: The development of the city was impressive.
* Proper noun: We visited the United Nations Development Programme.
In the first example, "development" is a general term referring to the process of growth or improvement. In the second example, "United Nations Development Programme" is a specific organization, hence the capitalization.