Brand wars, basically. A few major brands have taken open pops at their competitors in the past, and essentially it tends to create an expensive 'gloves are off' p***ing contest where each escalates, trying to put the other one in their place for a while, but ultimately achieving very little. These days it's mostly considered a very poor idea to do it. It's also looked down on by most reputable firms and ad agencies as lazy and talentless. They often feel you get much better (and more 'bloodless') results by emphasising your own strengths rather than taking a dump on your competitors.