Why Can't A Through-The-Wall Cooling Unit Vent To The Outside?

The simple answer to this question is that they are simply not strong enough to handle those warmer and colder outside temperatures. Standard through the wall units such as the N'FINITY, WhisperKOOL SC, Wine Guardian TTW and Breezaire vent warm air out the back of the unit into the same area they are pulling in the fresh air to cool down the cellar. These units are built to handle a temperature differential of no more than 25 degrees. Since you want to keep your cellar around 55 degrees, any area that is above 80 degrees will not be suitable for these units to be installed. When the temps rise above 80, the units work hard to try and cool the cellar, but will be unable to do so resulting in a shorter shelf life of the unit and warmer temperatures in your wine cellar. And because there is no heating component in these basic units, once the ambient temperature drops below 50 the temperature in the wine cellar is at risk of doing the same. If you have an application where the unit must vent to a much warmer or colder environment, then you probably want to look at one of the ducted systems as they are built with more commercial grade parts to handle those sort of extreme temperature ranges.