This term comes from the division “travel cases“

Formerly a light weighted case made of textiles and reinforcing frame. In the 1960s the case was replaced by a construction mounted from two injection moulded or deep-drawn plastic case trays. In general the term hard shell case refers to all cases that consists of two trays which are made of a hard material, e.g. plastic, aluminium or wood. These constructions are very stable and can withstand rough handling. Because of the manufacturing technology and the required tools for production, these case type is mostly used as travel case.