It is to preserve the quality and taste of the booch. If left to cool down until room temperature i.e. more than 12 hours, the fermentation rate picks up again and the kombucha will start to ferment, causing it to taste a little more sour and may affect the taste as well.
You can off course keep them in an iced cooler or chilled bag when you are on the go, but do avoid letting them warm up to room temperature!