I think a good portion of it depends on what the authors submit to the magazine as well. German subject matter is popular because of the wide variety of types, and camo, that were utilized. Asthetically, much of the Allied stuff was bland, Olive Drab or Natural Metal are your basic choices with a few notable exceptions.
I know that as a writer and a modeler, when I do a project with an article in mind, I try to do something that's unique, to set it apart from all the other 'Tiger', Panther, or Bf-109 build articles that everyone else is doing. I'm not saying that building any of the common subjects is a bad thing, because I learn something new from every article I read, regardless of the model being written about. I just like to build stuff that's different from all the rest...but I have my share of 109s, Mustangs, Tigers and Panthers like everyone else...but I usually don't write about them, because everyone is tired of them.
It's safe to conclude that a high percentage of folks are building 'common' subjects, 109s, 190s, Tigers, Panthers, that the percentage of folks who write an article about these subjects will also be higher.
I imagine that its also tough to decide what type of articles to run. Sometimes folks provide a basic build article of an esoteric subject, or a really detailed article of a common subject. Which would be more popular? That depends on who's reading the article...newer modelers might like a detailed P-51 article, whereas others might think, 'been there, done that', and lean towards a basic article on the Republic XP-72 for example. The real gems come when someone writes a really detailed story on a subject that we know little about, but can achieve with a small amount of work.
So next time you decide to write an article about a model, pick something less popular than a P-51 or Bf-109. I'm willing to bet we all have that 'weird' model kit in our closet that we really want to build. As for me, it's a 1/32 scale Bristol Beaufighter in USAAF markings. Pretty airplane, nice colors, and something we don't see too often. Not overly weird, but it's not a Mustang or Tiger tank.
Be creative!!
Jeff