It's easier to animate a bunch of identical helmets than to try to animate faces for every single stormtrooper. Especially since it would end up looking weird because there is no way they would have a unique face for each trooper. Oh, wait a second, isn't that TK-236 from episode 9? Didn't he get thrown into the reactor?
Clone Wars splits the difference by having them as literal clones. Then you just give the unique ones haircuts, eye colors, or tattoos (or all of the above).
Clone Wars had a great advantage in the animation category because the bad guys were all droids, and the good guys were all clones, making it easier to cut and paste and reuse models.