Doctors are on higher levels then regular people are. When we need information about something that deals with our health or something else, they're there to tell us. Doctors tend to persuade us as their patients and we are then led to believe them.
In most commercials, famous celebrities and athletes are used to persuade and get the viewers attention, especially if they are a fan of the person. They then speak facts to make you believe them even more to get you to purchase whatever product they're advertising.
Another way commercials try to get your attention is by showing something that is sadness, laughter, or fear. The publishers want to win you over by making you feel pity for them or help them make a product a huge hit.