why does new battery go dead?

If this is something new and hasn't been happening in the past then something is going on. I'd fully charge the battery and have it load tested somewhere other than the Dealer who installed it. Most auto parts stores will do the test for free. If that checks out, make sure everything is off when you exit the vehicle. Make sure the interior lights go off and there's no map light or glove box light that's staying on. Most newer vehicles have battery run down protection that will shut off these things after a short time if you happen to forget. Also pay attention to the weather. Does the battery go dead after it's been raining? Or when it's cold or hot outside? If so this can point to a short that happens when something gets wet or expands or contracts enough for a bare wire to contact something metal and complete the circuit to turn something on. I once had a car that when it rained hard the interior lights would come on and kill the battery. Hope that helps! Jim