Here's why:
* Pronouns replace nouns. They include words like "I," "you," "he," "she," "it," "we," "they," "mine," "yours," "his," "hers," "its," "ours," "theirs," "myself," "yourself," "himself," "herself," "itself," "ourselves," and "themselves."
* "A feather in your cap" uses a noun ("feather") and a possessive pronoun ("your"), but it doesn't have any words that replace nouns.