And the reason is vigilance. Vigilance means continuousness. Guard services offer 24-hour protection. Security-alarm monitoring companies don't go home for the weekends, and it can't be any different in the digital world. Attackers follow their own schedules.
Attacks often happen at inconvenient times.
Criminal hacking follows the academic year. Smart criminals attack banking systems Friday afternoon, after they've closed for the weekend. In 1973, the Arab countries attacked Israel during Yom Kippur, the holiest Jewish holiday of the year. If someone were going to launch a serious attack against a system, he would pick an equally inconvenient time. Vigilance means immediacy.