Everything kind of goes in phases, there's always an interest, it just may not be in fashion at the moment, so to speak. Even with fandom RPs, sometimes there's an oversaturation of a show or game or whatever, other times you'd be hard-pressed to find one. It can just be something as simple as the people who are usually into military RPs are busy elsewhere or just want a break.

Personally, I wouldn't mind trying a military RP at some point, but I'm honestly hesitant about it because I don't think a lot of people who would be attracted to it have the same vision as I do, which is historical accuracy, setting appropriate themes/ characters/ technology/ etc., and a tastefully done story that is respectful of the battle, war, and soldiers who fought. I want to tell the story of someone who was living through those events like historical fiction, not play text based Call of Duty, which I'm all but certain most people do where everything takes a back seat to action. Almost every military RP I've seen has had people arguing over their squad role (ten bucks says it's usually over who gets to be a sniper, even though very few people really understand how a sniper operates or what it's like to shoot at a distance) and expressing discontentment over the weapons they get to use. 
It's just one of those genres that I think attracts people who really aren't mature or knowledgable enough to handle in a manner I'd find appropriate because their interests are from video games instead of books and documentaries. 

To be clear, I have nothing against people who want to have an action oriented military RP, I'm just saying I'd prefer to have a game about conscripts in Stalingrad that actually reflect the historical accuracy and human element of men fighting a war more so than Ivan Drago killing a plation of German soldiers with an assault rifle that wouldn't be fielded for another year he stole from a dead Nazi.