Military uniforms have come a long way from the days when their main function was to identify which side a soldier was on while intimidating the enemy. In the old days, uniforms may have been impressive and colorful, but they were also uncomfortable, hard to clean and maintain, and not very practical for either combat or everyday duties.
Today, uniforms are becoming increasingly high-tech, with digital camouflage, protection against biological and chemical agents, blast protection, and even self-heating gloves. The frustrating thing is that the armed forces with global projection, like the US Army, ideally want uniforms that will be comfortable in all climates, are easy to wash yet durable, are fire retardant, and can repel insects.
Add to that the fact the US government is keen to keep as much of the supply train as possible on domestic soil while keeping the uniforms inexpensive and eco-friendly and things get a bit more difficult.