I answer this question by the economics of the content basically. Commercials pay more because the upside of the piece of content is much greater than the cost of it. If you do a worldwide commercial for a popular brand then they are going to make, realistically, a ridiculous multiple on the amount spent on the commercial, even if they go 2 days into overtime for everyone. Also the complexity of these projects is usually higher.
What I would call semi commercial ventures like indie feature films try to just make enough money to get more investment and so go on the same path as a commercial in as long as they know they can sell for X then they can pay Y, so as a film gets better components and people then they pay more and make more.