This is why ACUs have buttons on their pants and a zipper on the blouse
The U.S. military's uniform history is one of tradition and tactical purpose. Many tiny details on our uniforms date back centuries. The different colors in the Army's dress blues are a call back to the days when soldiers on horseback would take off their jacket to ride, causing their pants to wear out at a different pace. The stars on the patch of the U.S. flag are on the right as if we're carrying the flag into battle.
It took me six and a half years in the Army to learn that this symbol is supposedly an "A."(Photo by Senior Airman Kimberly Nagle)
Something that always stuck out was why the ACUs have the button and zipper locations opposite of civilian attire. Everything had buttons until the M1941 Field Jacket had a zipper with storm buttons on the front. Shortly after, many parts of the uniform would start using zippers as a way to keep them fastened. The zippers, like many things in the military, were made by the lowest bidders until the introduction of the ACUs in '04.
Zipper and velcro? But why? (Photo via Wikicommons)
The zipper on the ACU blouse is heavy duty and far more durable than zippers on a pair of blue jeans. The zipper is useful on the blouse for ease of access. The top having a zipper allows medical personnel to undo the top far easier than searching for a pair of scissors or undoing all of the buttons. The hook-and-loop fasteners (Velcro) is to help give it a smooth appearance.
Even OCP still kept the buttons, but added the sh*tty velcro back to the cargo pocket (Photo via wikicommons)
Buttons on the trousers serve a completely different purpose. The buttons keep them sealed better than a zipper. Think of how many times you've seen people's zipper down and you'll get one of the reasons why they decided to avoid that. Buttons are also far easier to replace than an entire zipper and a lot quieter when you need to handle your business.
Dress uniforms take the traditional route to mirror a business suit. The Army Aircrew Combat Uniform is on it's OFP.