It's not just military history, but other 'dead white guy' topics like Classical history (see Victor Davis Hansen). Most blame 'liberalization' to some degree,

It's not just military history, but other 'dead white guy' topics like Classical history (see Victor Davis Hansen). Most blame 'liberalization' to some degree, e.g. Vietnam making military topics less 'glorious', but I think the real reason is the rapid growth of universities since the 50's. There are probably the same absolute number of military historians as there were in 1920, but there was a tremendous growth in non-traditional historical fields as schools had to quickly add new classes and departments to keep up with the demand cause by increased attendance.