Short answer:

Good military history tends not to be good history, and good history tends not to be good military history-- their aims are different. Academic history is a bit like a treatise on aerospace engineering and finance; military history is more like a crash site investigation; one is about processes and institutions more than events, the other the converse.

Discussion:

Military history has a (often unfair) poor reputation in the academy-- frequently dominated by official histories reflecting institutional interest, it's history which often serves a public appetite with amateur efforts, not least of them those of the participants themselves; accounts of commanders since Caesar have been popular reading. I happened to go to grad school at one of the relatively small number of well regarded military studies programs at a civilian university (War Studies at King's College London), but it's more focussed on political science than history. For the most part academic historians look at bookstore shelves groaning with biographies of Stonewall Jackson and Erwin Rommel, random assortments of anecdotes and color without much in the way of historical technique and despair at the unfairness of it all.

There's nothing special about the World Wars in this regard-- consider French history as an example. There's an extraordinary abundance of scholarship leading up to and including the French Revolution-- but you get to the Napoleonic Wars and where's your Timothy Tackett, Francois Furet or Richard Cobb? They're just not there. Occasionally someone like Isser Woloch will venture into the field of Napoleonic history as an extension of the Revolution, but it's unusual to find what you'd call academic history written about popular topics in military history.

Part of it is that military history is dominated by "Great Men" and chauvinism styles of writing, and seldom attracts the same kind of scholarship that you see applied to civilian history. The social origins of Napoleon's leadership clique would be a great topic, and Woloch has a rare effort in this regard in Napoleon and his Collaborators, but its striking just how differently historians look at war as a historical process than do military historians. Perhaps the best illustration of the distinction is the celebrated work l'Étrange Défait ("Strange Defeat"), by Marc Bloch, one of the important innovators of historical method in the 20th century-- who also happened to be a veteran of the First World War, and fought in the French Army till their defeat in the Second, and then was in the Resistance (he was captured, tortured and shot-- along with his brilliance, this makes Bloch a hero to many, the scholar-soldier-patriot)

Bloch's book stands as a most unusual look at armies and warfares as institutions in a historical process. Bloch talks about, for example, the complex patronage networks within the French Army, and the ways in which that inhibited military efficacy. Typical military historians are good with after action reports, but tend not to be as tuned in to patronage networks and "the business of making an army run" in the way that academic historians are; note how few books there are on military logistics or promotion policies. When historians _do_ write about military topics, like say conscription, they do so in a fashion that doesn't tend to interest military historians or their typical readers; any of Napoleon's Marshals is a more profitable topic than the mathematician and revolutionary who supplied his army with conscripts and who essentially creates the levée en masse, Lazare Carnot. Carnot falls between two stools -- too military to be interesting to academic historians, too much of a social historical topic to be interesting to military historians. In the end, it's the mathematicians who do the most to recall Carnot's work.

When academic historians do write military history, they tend to do so with an eye on very different sorts of things and often break exciting new ground-- William McNeill's The Pursuit of Power stands as a rare example of Chicago School meets military technology, when I first read the book it struck me "why is there so little like this?"; the answer being that it crossed domains and was without an academic constituency.

I should note World War I did have an academic constituency, but not in the United States. It was likely of the most academic interest in Germany, where scholars looked at the history as a political science problem, whether its possible to escape the "Thucydides Trap". I'm thinking in particular here of people like Fritz Fischer and Immanuel Geiss; German academic history was more sympathetic to military topics generally, and the notion of studying the policies of "great men" like Bethman-Hollweg and Moltke as "history" had more academic legitimacy.

Similarly, well trained historians don't necessarily have the tools to gather and interrogate data about just what happens on the battlefield; a good historian may not be a good military historian. Part of what makes [some] works of military history "good" are disciplines that many historians don't have training in, like military science, sociology and anthropology. When you get to the "why does a unit break or mutiny" question, or "why do soldiers commit atrocities?", there are books like Simon Harrison's Dark Trophies: Hunting and the Enemy Body in Modern War or John Keegan's The Face of Battle that examine the behavior of men under stress together with kinds of data and frames of analysis that are less familiar to most historians.

Finally, there is the basic problem of what to say about battles. You can say who won; you can say why; but these brief thunderclaps aren't so much history as events. The coming of the Second World War in the Pacific -- that's a complex and multi-stranded examination of Japan's growing militarism, US colonial policy in the Philippines, attitudes towards China and much more-- but when you get to December 7, 1941 what is there to say? You can say "this is what happened", and tell the story more or less skillfully, perhaps with new archival information, but even in a work as well accomplished as Gordon Prange's (At Dawn We Slept), what is there for the historian? What do we understand better of the way the world works, of how communities of peoples formed up into polities build institutions to make war and find that they do or do not do their bidding? Writers and their readers tend to be interested in the tensions between the "Treaty" and "Fleet" factions in the Imperial Japanese Navy, or the superiority of the Type 93 torpedo, but rarely do both subjects engage them.

So for good and ill there are a lot of folks who write about Waterloo, who ponder counterfactuals -- where was Grouchy when he was needed?- and they're quite different to those who study the rot that undid the ancien regime and plunged what had been the richest nation in Europe into more than a decade of war, toward that end pursuing deep inquiry into institutions and social affiliations that were and how they came undone. The former can earn a surprisingly good income writing for their supper, the latter get tenure . . . sometimes the two domains do cross, but we clearly don't have as many James McPhersons and Paul Kennedys as we do writers of a more popular and less scholarly flavor of military history.

Sources

Bloch, Marc L'Étrange Défait
McNeill, William H. The Pursuit of Power
Prange, Gordon. At Dawn We Slept
Geiss, Immanuel. *Juli 1914. Die europäische Krise und der Ausbruch des Ersten Weltkriegs.*
Fischer, Fritz Griff nach der Weltmacht: die Kriegszielpolitik des Kaiserlichen Deutschland, 1914–18
Lazare Carnot ou le savant-citoyen , éd. par J.-P. Charnay - Paris, Presses de l'Université de Paris-Sorbonne, 1990