O negative blood is considered very useful because you can basically give it (fairly) safely to anyone whatever their blood group. Blood group O negative is what's known as the "universal donor" group. It's saved a lot of lives because it's given in absolute emergencies where one doesn't have time to check the blood group of the victim and get specific cross-matched blood which is safe for them.
Hence it's a blood group that is always in good supply, because it's so important in life-saving situations.