Here's why:
* "Mono" can be considered a prefix meaning "one" or "single", but it's directly attached to the root word "tone".
* "Tonous" is the root word, meaning "sound" or "tone".
* "ous" is a suffix, but it doesn't change the meaning of "monotonous" significantly. It simply makes the word an adjective.
Therefore, while we can identify elements like "mono" and "ous", they don't function as independent prefixes and suffixes in the way they do in words like "un-happy" or "care-ful".
"Monotonous" is a single, unanalyzable word that means "lacking in variety and interest; dull and tedious".