I think the big thing with WW2 RPs is less they're military but because being based on real events it can be awkward to try and keep true to the source for most people and usually requires a good deal of knowledge on the subject matter at hand. It's not enough you have a casual understanding of the topic, especially when the bar for such a setting is raised by experts in the field. And this is true also for alternate history, where the believably and organic nature of the plot and setting can hinge on how much one understands the circumstances that lead to the alternate history. Otherwise it can become forced and silly to anyone who has any serious appreciation or knowledge of the matter at hand.

On the surface it doesn't mean much to the casual RP'er. But it still doesn't rule out that an RP like that will likely attract people who specialize in that era or are big fans of it. And this can steadily raise the informal bar of participation. Though you may outwardly welcome anyone to try, there may still be a subconscious concern of people going to join when the OOC and general chatter is filled with high-level amateur or professional debate and argument over certain aspects in t he RP's setting.

So that may be a contributing matter in such a topic. And also a death-nail in it: too much talk and peacocking and establishment in the OOC to the point people spend all their energy talking about they're going to do X and Y so that when it comes to IC they have no energy to write about it, or the ideas have lost their magic. But this isn't strictly related to war-RPs or even WW2 RPs as a whole.