I have noticed this, too. I used to be a member a while back, and I recall there being a ton of WWII roleplays at that time. I have been craving a WWII RP recently, so I re-joined but I haven't seen a single one for over a month! While there are a lot of "Military" tags on roleplays, it is generally not a military-focused RP. There might be some characters who are soldiers, but that is not a main theme. 

Perhaps one reason for a lack of war-themed roleplays is that it is simply not a popular theme in the media right now. We don't have many war movies inciting folks to take the story into their own hands. Perhaps others do not like romanticizing war or living it vicariously through a character. I personally would never wish to experience war first-hand, but I am deeply fascinated by the military and would love to play it out in a safe, fictional environment.

Actually, I have been considering starting a WWII-based RP, but I am not sure I could come up with a good enough plot to keep it going. I am also not an expert by any means, so there would not be a large focus on historical accuracy and the like. This may be appealing to some but not to others...Still, since there is some interest in war RPs here, maybe I'll give it more thought! :)