Why is chronic lymphocytic leukemia (CLL) most common leukemia in the West but so rare in Asia?
This paper should help get you started. The exact reason is not yet known, but is most likely related to germline genetic differences between the two ethnic groups, as Asian immigrants to the US continue to have a low incidence of CLL.

References:


KAWAMATA N, MOREILHON C, SAITOH T, et al. Genetic differences between Asian and Caucasian chronic lymphocytic leukemia. International Journal of Oncology. 2013;43(2):561-565. DOI:10.3892/ijo.2013.1966. 
American Cancer Society: What are the risk factors for chronic lymphocytic leukemia?