Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things.
Since "bees" refers to the general category of insects, it's a common noun.
If you were talking about a specific group of bees, like "The Honeybee Hive" or "The Killer Bees of Brazil", then those would be proper nouns.