Green text on a black background is very readable in normal lighting, yet doesn't assault the eyes if you are reading in a darkened room.
In addition, there are still people using CRTs. Green on black is often clearer than white on black or black on white, especially in a small font, on CRTs because it is essentially monochrome, and so cannot suffer from fuzziness due to color misalignment.
FWIW, greenscreen (and amber) CRTs used green and amber phosphors; color alignment didn't come into it, right? As I understand it, green and amber because they were cheap.
Correct, alignment was not a factor for single-phosphor tubes.
The eye is not nearly as sensitive to red and blue so those colors did needed higher energies. But I doubt that it was because the phosphors are cheaper. Computer terminals and monitors were very expensive back then, but commercial color television made some of the common components (like phosphors) relatively cheap.
Green was found to be readable and pleasant. It was the most common for IBM equipment (such as the PC monochrome display).
Amber is very visible, especially in bright light. It was popular too.
If I recall correctly, those green phosphors were long persistence, so they could have a lower refresh rate. Amber and white phosphors came along later when it didn't matter anymore because logic got faster. Some of the first terminals I used had core memory, so they still retained their contents when turned off.
Unrelated but interesting. I always wondered if it was just nostalgia, preference, or some other reason for green text. It makes sense with RGB pixels of course. Now what about orange? That's not monochrome ...