I'm guessing it's a combination of factors.
1) Political correctness/liability concern within organizations.
2) Lack of demand for the magazines. This can probably be addressed by several factors:
2a) Preponderance of the same information available online, before the magazines become available.
2b) Higher cost of publishing and distribution, resulting in a higher magazine price.
2c) Relative lack of consistent quality in the magazines. Articles are mostly just ads these days in many of them, with only maybe 5-6 pages actually having original/useful content.
2d) Too many magazines to chose from (resulting in some of the above phenomena) resulting in people just saying, "no thanks".
3) Finally, people just order them and have them delivered in the mail if they get them regularly. The magazine companies see the same amount of profit.
But Time,Newsweek,US News,Sports Illustrated,etc.all have web sites and message boards,all are going strong circulation wise, and all can be found in your local drug store.But not gun mags.
Time, Newsweek, US News, etc. are all liberal publications, for the most part. Liberals are stupid.
Kidding aside, there's a number of good reasons for that, I think.
And I believe Sports Illustrated has had some -significant- financial woes in the last 5 years. IIRC, they re-targeted towards the mid-teen market, and increased the 'swimsuit' issues/content significantly to try and compensate.
Also, those magazines not only have wider demographical interests, but they also have much more material to pull from. In all honesty, there's probably only enough actual content to fill a single (albiet, thick) gun magazine every month, but they get stretched out across a dozen or so, and they're all pretty thin compared to (say) People, which costs less than the gun rag.
And there is a never-ending stream of celebrity gossip and world news from which to pull from. You can miss several gun mags without missing anything (different issues cover the same political/social topics in serial, quite often it seems), but something like Time or US News actually does some fairly indepth analysis and reporting on things. You miss one, and you've missed a significant amount of news, if you don't really read the paper or watch TV.