No good reason. I was in Kabul for a visit with my mother and father, in 1971, as they traveled over many years from London to Singapore where they bought an island in Malaysia on the South China Sea.
I learned three things that the press never covers.
First we took a few days off to relax in the cool nearby mountains at an American Embassy lodge. It was night time and we got lost. It was only about five miles outside Kabul in the green hills. We found a beautiful empty palace and set up a tent for the night.
When we got up in the morning an Afghan was standing over us with an Enfield rifle and a very threatening demand.
My father reached (slowly) for the nearby dictionaries. The Afghani didn't speak Farsi, Pashto or Urdu. That is when I learned how many mutually un-understandable languages there are in Afghanistan. It is not one country. It is a tribal region.
Second, you can look at Kabul in a Google map today. As in 1971, it is a zone of flat mud houses, dirt streets, hardly even a town with a few two story buildings in the center. This is the capital of nowhere and it makes Tijuana and Juarez look like Manhattan by comparison.
Third, I saw nothing resembling industry or productive output. The markets were filled with old rifles, thousands of knives and machetes, rusty metal and every imaginable form of tack for camel transport.
What could a military protect? We can probably form military rings around a few towns and run convoys between them and have a few of our own strategic bases but I can't imagine any other military goals.