My guess.
I think "back when" (late 1940s and early 50s) it was decided that low band VHF radios were easy to build, easy to maintain, fairly rugged and had adequate coverage. And, later, since the equipment was replaced piecemeal instead of all at once, the new stuff had to be backwards compatible with the old stuff, so the "low band" became almost a tradition. Now, there are HTs out there that can do 30-512MHz in various modes and the frequency horizons can expand.
For tactical communications, no more range than needed is good. More range means more of a chance of being heard by "the wrong people" and even if the signal is encrypted, the presence of a signal can provide useful information.