easier to maintain, neat professional look (this DOES matter), less likely to be caught in something (branches, wire, etc.), stays out of your eyes, and significant Facial hair (like a beard) + Gas mask = you’re fucked because your mask is no longer capable of being air tight, long blonde/red/light brown (not so much this one) stands out like a sore thumb when in concealment.
Also to maintain a sense of uniformity.
On another note, a soldier represents their nation similar to how a corporate employee represents their company on a business trip. Thus the military tries to maintain a crisp, clean, professional look of the troops as our troops are, in a manner of speaking, representatives of their respective nations and as any corporation or company will tell you, unkept, messy, unclean employees are a BIG no no.
I would also like to point out that in Conventional warfare, troops on the front line do not have to worry as much about personal appearance and hygiene (though they are still very important). I remember reading a few stories and being told stories from veterans of time on the frontline such as an armor company in the Gulf War which spent it’s entire time during those 100 hours pretty much on the tip of the spear just going and going without pause. When they finally got to get away from the front, they were a bunch of grime covered, grizzly, smelly men. Of course they had to clean up, shave, and whatnot, but they made their first order of business to plop down and eat in the chow hall. Anyway, point is, when on the frontlines of a conventional war when you don’t have time to shave or whatnot, then you don’t. However, we’re not in conventional wars, thus once back on the FOB, you better get you and your gear nice and clean to the best of your ability.