What caught him was the mistake of using the 1944 date. It was a collector who got a 1944 nickel without the mintmark over the dome that started people on his trail. If he hadn't used the 1944 he probably would have gotten away with for a much longer period. As for why the others are so rare, most people know about the 1944 no mintmark, but few know about the other dates and in general who bothers with worn circulated Jefferson nickels? The war nickels sure, the 38 and 39 D & S, and the 50 D yes, but the rest of them? They are too low a value for most anyone to bother with. so for the most part no one looks at them. And if they did they have to know the markers to look for.
My spouse and I have found a rate 1947 Henning with a looped R. We were featured on Thursday Night Coin Q&A with Jeff Stanly and his panel