Here's why:
* Alliteration is the repetition of the same consonant sound at the beginning of words that are close together. In this case, the "s" sound is repeated in "Susie," "saw," "seashells," and "shore."
* Onomatopoeia is the use of words that imitate the sounds they represent. There are no words in this phrase that imitate sounds.
Let's consider an example of onomatopoeia:
* "The crash of the waves against the shore." The word "crash" imitates the sound of waves hitting the shore.