Short people require less calories, protein, space, and are harder to hit with bullets and arrows.
The environment selects for short genes (and against tall genes) in times of food scarcity, because people with tall genes require many more calories and protein, and being calorie-deficient and protein-deficient while growing up with tall genes results in poor development (insufficient food intake) and higher likelihood of diseases (the immune system gets compromised by insufficient nutrition).
The increase in europeans' heights is correlated with (and very likely caused by) increasing abundance of nutritious food, thanks to the development of more productive farming techniques during middle ages, and even further accelerated during the industrial revolution.
Nowadays, only primitive tribes and some societies where food is scarce face selective pressure against tall genes. North Korea is one such example.
