One issue is that, if a magazine or newspaper includes a lot of graphics or photos, these can be expensive to transmit to your Kindle over the 3G network, and Amazon passes those costs on to the publisher, who would have to pass those costs on to you or else leave out a lot of those images. (cite)
Other issues may be that it takes more time and effort to format a magazine’s content so that it’s readable on a Kindle than the publisher wants to spend, or that the publisher would prefer you to read the print version so that you get exposed to all the ads, which help pay for the content. But this is just speculation on my part.
If you have the (free) Calibre program, you can use it to download some free magazine/newspaper content.