EPP stands for Expanded PolyPropylene and seems similar to EPS (Expanded PolyStyrene). EPS is best known as the white foam packaging material. EPP is usually black and well known for its use in bicycle helmets. EPP is somewhat costlier than EPS but stronger. Breaking the product or peeling the surface is much more difficult compared to EPS.
Molds are not as expensive compared to injection molding while there are many more geometrical options. The material is soft which means undercuts may be allowed up to a few degrees. Also the material is quite 'forgiving' which makes engineering the interfaces more secure.