Here's why:
* Parts of speech refer to categories of words based on their grammatical function (noun, verb, adjective, etc.).
* Haiku is a style of poetry with specific structural rules (three lines, 5-7-5 syllable count).
You could say that a haiku is made up of nouns, verbs, adjectives, etc. But haiku itself is not a part of speech. It's like asking what part of speech a sonnet is.