|Quoting ANother (Reply 1):|
Brits are more willing to pay more for quality (as I am) whereas in N. America it's the price that is most important.
On that note, many (if not most) magazines in the US are primarily advertiser-supported in order to keep subscription rates and cover prices down, so many magazine operations may be limited by that right off the bat.
However, having worked at a magazine company, I think a good answer is planned obsolescence. If you look at most weekly or bi-weekly periodicals (Time, Forbes, &c), the paper is thinner and the construction weaker overall because these magazines know that their readers will discard them almost as soon as they've finished reading; why invest in expensive papers and inks if it'll end up in the trash days after being delivered? Compare that to a National Geographic, whose very construction and heft signifies permanence because people want to and will keep it.
Finally, on the content side, I will say this: America has too many magazines. We have magazines for almost every niche and even then, there are often two or three *competing* against each other. There are simply too many rags and not enough journalists who care to earn crap money to write 12 times a year (which is why so many of them freelance). That disparity inevitably leads to thin issues overflowing with full-page ads, space-consuming graphics and tables and sponsored gate-folds. I don't know if the U.K. has the same problem, but I think if 500 or so extraneous magazines were to shut down, we may very well see an increase in the quality (at least content-wise) of the periodicals.
When you do things right, people won't be sure you've done anything at all.