Green on black terminal windows are the way they are for the same reason old oscilloscopes and radar displays were green on black - it's more cost effective to make a cathode ray tube that glows green. For a long damn time, all terminals came green-on-black, simply because that was the cheapest way to pair a CRT with a keyboard, and hardware terminals were what they used back before PC's were popular. Or invented.
The result of this was horrific eyestrain. Yes, some people can handle bright colored text on a black background. Most get eyestrain or worse, migraines. This is especially so if you switch from green-on-black to black-on-white (like a printed page).
Typists and transcriptionists and grad students and pretty much anyone who needed to refer to a printed reference hated it. In the early '80s, color monitors were pretty much crap for text (too fuzzy, not enough resolution) so there was a boom in the production of "amber" monitors. These used monochrome CRTs that phosphoresced a muted yellow-orange. This wasn't quite as jarring to the eyes.
Then someone came up with paper-white monochrome CRT's, and that was pretty much all she wrote for greenscreens.
Geeks keep it alive, because of nostalgia and tradition. It's looks high-tech and cool, because there was a time when it was high-tech and cool - and because there is an association with Unix, and by extension, Linux. What's more Unix than a DEC vt100 terminal hooked up to a PDP-11? Nothing. That's about as close to the metal as you can get without a soldering iron.
But, please, for the sake of your eyes and the eyes of others, don't pretend there is any inherent advantage to green-on-black for the vast majority of users.
