Because most newspapers are written at about a 7th-grade reading level. That's the reading level of standard informal American English. Anything higher than that is most likely inappropriate for corporate use (though obviously not for academic reading or medical/technical materials).
Anyway 7-8 is the target; I've not really seen a lot of stuff get produced in my work that is actually that low. It's usually more like 9-11. What we really have to watch for is the percentage of sentences using passive voice.
I know that in the protocols for our clinical research studies, the informed consent form (which attempts to explain the study rationale, and its potential risks and benefits to participating patients) is supposed to be written at an 8th grade reading level. Anything fancier, and you couldn't be sure that all patients would have understood the risk adequately.
(Actually, having the patient understand the risks adequately is an impossible goal - nobody ever really does, and there is usually far too much anxiety and emotion floating around, particularly with seriously ill subjects. Plus humans are very bad at assessing risk anyway - our brains aren't wired for it. You'll get different enrolment rates if you say:
the data to date suggest 1 of 4 people receiving this treatment will die
the data to date suggest 3 of 4 people receiving this treatment will survive
though, obviously, both describe identical scenarios)
Isn't there something strange about the fact that virtually everyone in America today is expected to complete high school, and yet newspapers, corporate documents, and other kinds of document don't expect people to be able to read above an eighth-grade level. I certainly approve of the kinds of protocols sionnach mentions, where one wants to be sure the patient understands what is going on. But generally I think the American media do far too much "talking down" to the public. The result is that when someone displays the least erudition or even the ability to speak articulately and knowledgeably on a subject he or she is accused of "elitism". What's wrong with expecting people to use a dictionary if they don't know a word? Isn't that more respectful than assuming they won't understand a twelfth-grade-level text?
Rolig, I have no real answer for your question except a couple of thoughts that might apply. And I don't mean to sound argumentative, so I hope you (and anyone else) won't take offense.
1) The American penchant for irritation with perceived "elitism" has a long and ... I was going to say "glorious" but let's face it, it's more like "violent"... history. We are a nation of people who wish we were tough frontiersmen (and -women) who "ain't got no use for book-larnin'." (I coined that phrase. You're welcome.) We pride ourselves on being tough, strong, resourceful, inventive, profitable... Not necessarily on being brainiacs.
2) Not everyone does complete high school.
3) Not everyone who completes high school can read.
4) Of those who complete high school and can read, many do not see reading (even something as simple as a newspaper) to be a fruitful or interesting hobby, or worth their time. (I could give you a recent, startling example from my own life, but it would probably bore you.)
Consequently, things like newspapers, forms, and tax instructions are to be written at an "average" reading level to ensure maximum understanding.
This is not to say that more erudite materials are not available to those who want them. But for the purposes mentioned on this page, you want the maximum number of people to understand. And for that you use an "average" reading level.
I don't see anything particularly harmful about this practice. In fact I find it egalitarian and, in that sense, quite American.
c_b: As a great-great, etc. granddaughter of a DAR, I know exactly what you are saying. Americans are 'git down to business' kind of people. Always were. And to those who came after, you're welcome.
As a 21st century mother of two children, I also know what you are saying. We read to our children from the time they were still wet behind the ears until they kicked us out of their beds at "reading-time" (about 12 or 13 years old *sniff*). I always was a BIG reader, my husband, 'meh.' Our kids have soooo many more distractions (not that it's all a bad thing...especially girls' sports) than we had, it's not surprising that they don't read as much as some of us older (gulp) folks do. But I firmly believe that they WILL -- IF we keep the books around, talk about the books, refer to the books, love the books...etc.