Here's why:
* Prefix: A prefix is added to the beginning of a word to change its meaning. There's no part of "useless" that can be separated and understood as a stand-alone prefix.
* Suffix: A suffix is added to the end of a word to change its meaning or function. While "-less" appears to be a suffix, it's not used independently in the same way other suffixes like "-ing" or "-ly" are.
"Useless" is a single word formed from the base word "use" and the suffix "-less". The "-less" part means "without" or "lacking", so "useless" means "without use".