Here's why:
* Arthropods have a hard exoskeleton made of chitin, jointed legs, and a segmented body.
* Mollusks have a soft body, often with a shell, and a muscular foot.
Hermit crabs clearly fit the description of arthropods:
* They have a hard exoskeleton.
* They have jointed legs (ten in total).
* They have a segmented body.
While hermit crabs do use empty mollusk shells for protection, this doesn't make them mollusks. They are arthropods that have adapted to use a unique form of shelter.