IOW, combat boots seem to offer more protection against twisting an ankle; the downside seems to be a slightly less biomechanical efficiency of walking. Why is that when the usage profiles would be very roughly similar?
hiking and combat may involve similar loads, similar backpacks, similar terrain.
But one thing ain’t similar: hikers pick their own boots, sitting a comfortable chair with a polite and soft-spoken salesman.
And they dont even have to polish 'em every morning, either. 
There’s a fairly wide range of what could be called ‘hiking boots’, from trail running shoes up to mountaineering boots. The biggest factor for hikers* : weight. People are almost certainly going to go for a balance of protection and comfort – on a long hike, the weight of the shoes can make a huge difference. The oft-quoted saying is “one pound on the foot = five pounds on the back”. I would almost go so far as to say it’s higher than that, more like 10 lbs.
*From a design perspective, but often as not from the consumer’s point of view.
Combat boots aren’t just deigned to support the ankle, but also to protect the leg from casual injury while traversing a wide range of different environments. True mountaineering boots–as opposed to casual hiking and backpacking boots–have ankle support and protection similar to or exceeding that of issue combat boots owing to the need to protect against injury while technical bouldering or traversing. Much of what passes for “hiking boots/shoes” today are really little more that a robust high top cross-trainer which are more comfortable for casual dayhiking but lack adequate support for backpacking and mountaineering.
Stranger
And I suspect that the typical army boot is designed to be able to be worn all day long and keep the mud, and sand out of your socks while givining you pretty good traction. Nothing more.
Hence the proliferation of lowtops, and ankle sprainers with actual, by design, curved soles that make it easy to fall over on even the flattest surfaces.
Many of the boots sold as hiking boots these days seem to be made to look cool, rather than function as a boot.
I can vouch for this. I have some army boots and after a few hours walking through the dunes and playing with a kite on the beach in Aberdeen, my brother’s shoes had a considerable amount of sand in them whereas my boots were sand-free.
That’s why my mother wears them.