Like it or not certain accents tend to be stereotyped in our minds. Eg gentle Scottish accents often used in financial/insurance ads because we associate them with financial savy(tightness). Scousers in an ad would make you think of being robbed, therefore not good for the face of a retail outlet trying to place itself as a bastion of good value in the grocery sector. If I was paying three figure to an ad agency I wouldn't want them to assemble a cast that made the finished ad seem like an episode of Jeremy Kyle or boys from the black stuff tbh.