It's probably a throwback to the days when they had to record everything by paper and the market was totally reliant on people doing the actual trading. People do have to sleep and rest afterall.
Back in the day when I was a low level analyst i asked this question of my boss, his answer was because if markets were open 24/7 you would never get to go to sleep.
Biggest Market in the world is the FOREX market, which is currency trading, and they trade 24 hours a day except for a few hours on Sunday. Their open exchanges move around the earth each day with the sun. So you can trade at night if you so desire.