Whenever there is a pandemic-level event, officials will try to quarantine an area to prevent further spread of a virus. We have seen this happen both in fictional stories and in real life. In either case, something always seems to go wrong. Part of the problem is that viruses, by nature, are unpredictable. They find a way and the nature of man to panic only seems to aid that fact. There are always sure-fire ways to stop a virus but they always involve consequences we (in the real world) are not willing to accept.
Things are different in the world of Dying Light. The city of Harran is essentially lost. The quarantine is preventing the spread of the zombie outbreak but officials know that it is like a band-aid on a bullet hole. The only real solution is to erase Harran, literally. Whether there are uninfected survivors inside the city or not, the rest of the world cannot afford to let what lurks in the city get free. The city is on borrowed time and much like the survivors inside, it may not see another sunrise. . .
Good night. . . Good luck.