If he was not the type of person who owned property and would be in a certain area for any length of time. It is highly likely he might not have been counted. He could be in transent when the census is being taken.
If he was in the military for a long period of time and according where he was statione. he might not be included. They do take the census in a given militery area in some cases; but that is not always true; because you can't list some military establishments in a census and if he was not counted in his home. He would miss those census.
I know someone who is in his sixties. If he was not counted when he was a young man when he was living with his parents. Then he would never show up on any census. He had a life of hopping and skipping around and didn't want to be counted in the census.
It can be 101 different reasons.