They're catering to their audience. It just makes good business sense (which isn't necessarily a bad thing). A glance at these boards will show that the majority of people here build military models and I believe this is a good cross section of the modelling community. Realize that this statement is coming from a modeller who doesn't build military models. Also, most dioramas that I have seen are of a military nature, and because of their very nature also make great magazine covers. That's not to say that FSM doesn't cover different subject matter as I've seen my favorite subject (airliners) at least mentioned almost every month along with the occasional workbench review.
The publishers have been pretty upfront about the fact that the articles that get submitted are the articles that get published. They're not written by "professional" writers, just by modellers who thought it would make a neat article. So, want to see more [insert favorite subject here] articles? Write one up and submit it.
Dan (the hypocrite)