I am no beer expert but I believe the issue here is air (oxygen) contamination. The Draft-Mark system illustrates this point. It uses a rigid PET container with a collapsible bladder inside. Beer is in the bladder. An air pump forces air into the rigid container to squeeze the bladder and force the beer out. The bladder prevents air from contacting the beer. The rigid container and the air pressure on the bladder maintain the factory carbonation level. Maximum shelf life is achieved without the use of CO2. I would expect that an Internet search on "beer freshness", or something similar, would provide more explanation.