Here's why:
* Common nouns refer to general things, not specific ones. "Nano" refers to a unit of measurement, which is a general concept.
* Proper nouns are specific names, like "John" or "New York City".
* Collective nouns refer to groups, like "team" or "herd".
* Abstract nouns refer to ideas or concepts, like "love" or "freedom".
While "nano" can be part of compound words like "nanotechnology" or "nanometer", it doesn't function as a proper noun in these cases.