Why is night vision green?
 

Because they ran out of purple.

Because yellow/Green is the “most visible” color perceptible to the human eye. Right in the middle of our visible spectrum, less people have trouble perceiving it and the majority of people will perceive contrast among yellow/green than any other color. Same reason safety clothing and emergency vehicles are often colored this bright yellow/green.

I often wondered why hunters are required to wear blaze orange when this bright green stands out just as well. I guess some would think it looks more like foliage! I've never seen any bush, tree or grass that color green.