HDD is not suitable for exposure to environmental forces
HDD mechanical system does not like shock, vibrations and uncontrolled temperature environments. Low, high and fast temperature changes impact adversely its operation. Shock and vibration experienced in many mobile applications would prevent HDD from operating. One does not have to quote military applications. The up-scaled laptops today include add-value shock detector that automatically shuts-off the HDD to prevent its damage.
Special ruggedized enclosures can be built. They drastically increase HDD deployment cost that most likely would exceed the cost of solid state memory.