Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized. For example, "John," "London," and "Amazon River."
* Common nouns are general names for people, places, or things. They are not capitalized unless they begin a sentence. "Pecan tree" is a general term for a type of tree.
While "Pecan" can be used as a proper noun (like "Pecan Grove" or "Pecan County"), when used in the phrase "pecan tree" it is a common noun.