It turns off the thirst mechanism and athletes stop drinking before they're fully rehydrated. Water is also a natural diuretic. It signals the kidneys turn on and produce urine. They contain small amounts of sodium and glucose. This combination has been proven to promote re-hydration.
Sodium turns on the thirst mechanism and athletes are more likely to keep on drinking.
The small amount in a sport drink will promote re-hydration but not excess fluid retention. A Good Sport Beverage on the WIAA web page.