Here's why:
* Alliteration is the repetition of consonant sounds at the beginning of words (like "Susie Seashells").
* Onomatopoeia is the use of words that imitate the sounds they represent (like "buzz," "crash," or "meow").
The phrase "Susie Seashells" doesn't imitate any sound, but it does repeat the "s" sound, making it alliteration.