There made a lot of those planes. If there was indeed a defect then it's not beyond reason that there would have been more than one plane that had the same problem.
They made a lot of 767s, too, but to my knowledge only one has been torn apart in mid-air when its thrust reversers deployed. They made a lot of DC-10s, but only one that I'm aware of had a its cargo hatch fail catastrophically or have an engine tear off. They made a lot of MD-11s but so far as I know only one was brought down by an electrical fire.
Civil aircraft are well designed, well tested, and generally carefully manufactured and maintained. When system failures occur, as the will with any manufactured product, they are rare and usually don't occur again because a solution is identified and a corrective action put in place. The fact that a one time failure could cause a fuel tank to explode under the right conditions doesn't surprise me much, and the fact that it hasn't happened again doesn't surprise me at all. It just means that recommended corrective action has been taken.