Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they start a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
For example:
* Common noun: dog
* Proper noun: Fido (the name of a specific dog)
Since "beggar" refers to any person who begs, it's a common noun.