The Jaffa armies from Stargate SG-1 offer a similar conundrum, but one whose explanation can be applied here. The Jaffa were subjugators and the muscle behind an institutionalized society of worship for their Goa'uld masters, which began with the use of superior technology against against bronze age weapons and armor. They never fought wars against non-Jaffa armies or real organized military forces, and when they did (re: the Asgard, other advanced races, etc.), they generally were on the losing end as the opponents were advanced to the point that they didn't HAVE to fight them openly. In the case of the Jaffa, their only really measurable foe was each other when a System Lord went to war against another one, putting them on equal terms more or less. It's no surprise then that all their big fancy armor and energy weapons meant to intimidate and terrorize the backwards god-fearing masses, ended up being useless against Earth's bullets, but more importantly the regimented training and battle tactics of the USAF and Marines.
For the Empire, at least by the point of the OT, it was the same thing. There was no real enemy to fight, and there was up to two decades of time where the job of a trooper was to subjugate and terrorize rather than fight in any military fashion ("Rebels" events nonwithstanding). Stuff like OT stormtrooper armor may have been cheaper to turn out for the disposable soldiers, and such things as actual resistance to weapons fire or peripheral vision taking a back seat to the fear factor of a battalion of walking iPods advancing on the average backwater planet bumpkin, and having the sheer numbers around to pull off what you need to do. Enter the Rebel Alliance, who are making a stand with a mishmash of weapons technologically, but the motivation to get organized into a fighting force that the Empire was not equipped to fight even with superior numbers.
As to how this ties into the evolution of the armor, perhaps the relative awesomeness of the PT clone armor gave way to a generation of cheap, weak armor meant more for psychological warfare over interplanetary conquest. They couldn't make a full-scale change in four years by the time Palpatine kicked it. Fast forward to TFA, where a new force of Empire types is ready to do conquest again. Good thing this dovetails nicely with the relatively simple look of the OT armor versus the glitzy CG clone version, and the glitzier ones we see now that benefit from the current state of movie propmaking...
Mark
