Tactical gear is often considered to be derived from military use, because what could possibly be more tactical than the military? However, in the case of pants, they had a very different tactical beginning.
There is no doubt that much of the military gear after WW2 specifically began being designed with a military perspective first rather than just being gear. For example things like hats, jackets, shirts, gloves and pants were all being looked at differently, from a military perspective first rather than a civilian one. Up until WW2, most gear was simply civilian level gear that was slightly adapted to be used my the military. For example, clothing being dyed in camo colors but still retaining all the basics of civilian level garments. For pants, the biggest change was adding cargo pockets over the thigh areas and starting to steer away from predominately wool and cotton materials.
Tactical pants however, had the most dramatic change thanks to mountain climbers who needed more durable outdoor apparel. As a result, they began adopting pants that were made with lightweight materials, focusing on a blend consisting of 65% polyester and 35% poly cotton ripstop. Then they began modifying belt loops and making them thick enough to support a utility belt that had the added weight of climbing gear.
All of these changes made them very attractive to military services looking for an edge for their soldiers. Lighter, more durable pants mean soldiers stay warm and move around better which is a big advantage. Using ripstop materials and bigger belt loops meant carrying around a sidearm and other military equipment was going to cause damage to the pants. Both mountain climbers and military services then added other additional features such as reinforcing parts of the pants to ensure enhanced abrasion protection and also adding interior knee slots that can accept kneepads. At last, tactical pants were born!
Ben