Growing a shell is a significant material and energy investment for a snail. They have lots of predators which can easily eat them, having beaks which can pick them out or teeth strong enough to break the shell. Those which are not big enough to have a strong enough jaw, are small enough to be able to reach inside the shell.
Why do snails still have shells? Is there a large group of animals which cannot eat snails because of their shells, and would be able to if the snails had no shells?
I'm thinking of commonly known land snails.