I have to check sources on this but to my recollection, the biggest reason for stomach cancer historically has been infection by Helicobacter pylori, which for decades caused duodenal/stomach ulcers in many adults with doctors only treating the symptoms instead of the underlying cause (the bacteria), because they simply didn't know how common it was as a cause of this problem. After Barry Marshall and Robin Warren showed in 1982 how it caused peptic ulcers, treatments for ulcers changed and gastric cancer rates fell from what I remember. Those two doctors even won a nobel prize for their discovery.