Here's why:
* It's not a proper noun: "Enclosure" is a common noun, not a specific person, place, or thing.
* It's part of a standard phrase: The phrase "enclosure(s)" is a standard part of letter-writing etiquette and doesn't require capitalization.
Example:
> "Please find enclosed a copy of my resume."
However, there are rare exceptions:
* If "enclosure" is part of a title or formal name: For example, "The Enclosure Act" or "The Enclosure Movement" would be capitalized because they are proper nouns.
* If you are specifically referring to something named "Enclosure": For instance, "The document is in the Enclosure folder."
In most cases, though, "enclosure" remains lowercase in a letter.