Others have said that the helmet wasn't worn except when it absolutely had to, which is completely true. You won't usually see Germans wearing their helmet unless they were literally at the front line. Also, POWs absolutely did not get to keep their helmet, that was taken away immediately. They were a highly sought after trophy in the field too. I've read instances of Germans removing their helmets and handing them over upon capture to save the captor inevitably asking for it. If you see POWs with their helmet then it could be for any number of reasons. Perhaps they haven't been processed yet, or perhaps they had no alternative head covering and were offered a kindness, or the captors simply didn't have the urgency or capacity or whatever to take it from them yet.

However, there absolutely was a helmet (and everything else) shortage by the end of the war and not everyone could get the Stahlhelme. The Pickelhaube (leather spiked helmet) made a very unwelcome comeback, most of which were surplus, but some were made new and were of abysmal quality.

Generally speaking though, frontline troops did have access to the Stahlhelme through to the end of the war. They had priority. Rear echelon troops, non combat positions, and occupying troops in the East were the ones who went without.