How come oral rehydration drinks dont contain magnesium and calcium? is this not lost in vomiting and dihorrea?

Not necessarily: Re-hydration is a matter of returning the major players in fluid balance to the body. The sodium/potassium/chloride & bicarbonate ions that make up the primary players in keeping fluid within the blood vessels. Glucose is usually included as a fuel source. The other elements would be part of total nutrition, but not a fluid to return basic fluid balance.