Why brown rice takes more cooking time than white rice?

Brown rice takes longer to cook than regular polished white rice, about forty five minutes versus fifteen to twenty minutes, because of the structure of the brown rice grain.

The way in which rice is processed makes all the difference. After harvesting, the seeds are run through a rice huller/husker for milling to remove the outer grain husks. Following this process, the resultant is the brown rice. Thus, the germ and the inner husk (bran) are intact and the rice grain is unpolished.

Consequently, the germ and the inner husk contribute to the increased time in cooking of brown rice as compared with polished white rice, which is devoid of the germ and inner husk. 

Brown rice has far more nutrients as vitamin E, thiamin, riboflavin, niacin, vitamin B6, folacin, potassium, magnesium, iron and over dozen other nutrients. Added to that, the dietary fibre contained in white rice is around a quarter of brown rice. Brown rice may help reduce the risk of diabetes, heart disease and certain cancers, and may even contribute to maintaining a healthy weight.