First things first: all Kobe is Wagyu, but very little Wagyu is Kobe. The answer to why is quite simple: Wagyu means "Japanese cow," specifically Japanese Black, Brown, Shorthorn, and Polled genotypes, which are genetically predisposed to develop fat inside the muscle tissue. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.