The primary reason for baseline deficiencies in both the general population and in serious athletes is that it's difficult to get proper amounts of zinc and magnesium solely through diet alone. USDA studies show that 68% of self-selected diets contain less than two-thirds of the RDA for zinc  and 39% contain less than two-thirds of the RDA for magnesium.  While zinc and magnesium are contained in a wide variety of foods, it's been my experience that athletes don't acquire sufficient quantities through their normal diets. One reason may be that foods high in these minerals aren't necessarily the most desirable. For example, the best food sources for zinc include oysters and beef liver. These foods aren't consumed by most athletes, nor should they be.