Names were omitted from war memorials for a number of reasons. There seems to have been a view that there was a difference if death occurred outside the years of the war i.e. 1914 - 1919. Many who died slow, lingering deaths from wounds, poison gas or mental trauma were often not commemorated anywhere, not even in official records.
If your family had left the area or died out, so that no one remained who had information about you, you were very likely to be overlooked.