"What is the reason behind the increase in the number of patients with IBD in the Asian area occurring after that in the USA and western Europe?"

This important research question is quoted from "Is there a link between food and intestinal microbes and the occurrence of Crohn's disease and ulcerative colitis?" published in JGHF (2008)
Conclusions:  The present study supports that intestinal environmental factors, such as food and microbes, are very important for the pathogenesis of IBD. However, further studies are needed, because most studies lack direct evidence of a direct link to the pathogenesis of IBD.
From the Abstract:  We found several papers describing the positive association of animal meat and sweets and sugar with the occurrence of CD and UC. An analysis of Japanese epidemiological data suggested that the registered number of patients with CD or UC started to increase more than 20 years after an increased daily consumption of dietary animal meat and fats, and milk and dairy products, and after a decreased consumption of rice. Many studies implied a positive role of intestinal microbes in the occurrence of IBD. Intestinal environmental factors, such as Westernized food and intestinal microbes, seem to be involved in the increased occurrence of IBD. [emphases mine]

Why would the increase in cases be 20 years after?  Is it related to the extent of the perfusion of a westernized diet?  Or is something else at play?