Replacement drinks are (or should be) high in sodium because you don't sweat out nearly as much potassium.

I don't have any links handy, but it's easy enough to google. I did just look up sodium losses (anywhere from 550-2000 mg/hr, depending on the individual athlete and their degree of heat acclimation). Any of the other electrolytes are much less.

Drinks that aren't balanced are typically meant to be used as part of a cocktail of several of a manufacturer's products. Drives me nuts.