One reason is political consultants. A group of high paid people (usually paid for by a special interest group) offer their advice about what to say and do to get elected. You will notice in the past 20-30 years all of our candidates have the same style campaigning. Its a lot of ra-ra, 30 sec. bites that might make television, and practiced answered for debates.
Never do you see a candidate sit down and have a teaching session with a believable host, like Jim Lehrer or someone similar. During that session the candidate can explain how the economy works between government, capitalism, and the citizens of our country. It needs to be a message delivered at maybe a senior high school level, so the people who don't understand these concepts can understand and learn. But the focus should be on teaching, not on politics.
You also need a candidate that has the speaking skills to make this kind of presentation. I am not sure Romney or McCain could. Obama is a much better speaker - especially to the lower middle class.