Actually the reason for the overrepresentation is that they have a lot more disposable income per person than the average American, due to a combination of higher average education level and fewer children to support. Advertisers target people who can be persuaded to buy their products/services, not just random numbers of people. No point spending money to persuade people that they want your product/service, if they can't afford to buy it.
Forgot to include that part. Two incomes, no kids. That would imply that they also have more disposable income to donate to their favorite politician/political party and be more politically active too (no time constraints from children).