There are a few factors.
First, obesity. For Caucasians, they’re much bigger so their disease patterns are a lot different. For Caucasians in Western countries, it’s not so much stomach cancer but cancer in the food pipe.
For Asians, it’s more of the food we have and the genetic component we have as a race.
Smoking and alcohol intake is slightly higher. So these all compounds to make stomach cancer more common in our part of the world. Also, H. pylori infections are more common on our side of the world.