Elrathia was small and lived at the bottom of the sea, unlike its relative Carolinites which swam upside down near the surface. It was near the bottom of the food chain, because it was small and easy to catch. Elrathia lived in the exaerobic zone, where oxygen levels were so low that it would hurt most organisms.
Elrathia was defenseless, so it had to be as quick as possible. But even that would not help it escape anomalocarids, because anomalocarids were much faster and much more powerful than trilobites.