[Phantasm]

Also, IMO, the point of stormtrooper armor is not to protect the soldier; it's to turn the soldier into a faceless enforcer of the Emperor's will. Any protection it gives is secondary to that purpose.

Remember that the Empire initially treated the troopers as expendable; I'd hazard a guess that the stormtrooper armor was made by the lowest bidders, since they had to outfit millions if not billions of soldiers galaxy-wide.