Why? The DC-10 had flown for some time before the loss of a cargo door brought down the Turkish Airlines plane outside of Paris. And flew for years more before an engine fell off while taking off at O'Hare in 1979. The 767 had flown for years before the engine reverse suddenly activated in air in 1991. The MD-11 had years of flying time before the SwissAir flight was brought down by an electrical failure. Airplanes are well built and well designed. Problems can surface years later due to a single fluke combination of factors and bring a plane down.
At none of those other crashes were there dozens and dozens of credible eyewitnesses who saw something streak up into the sky and intersect with the path of plane at the same time it exploded.
Having both flown aboard aircraft as a crewman in the service and performed maintenance on them, I’m pretty familiar with maintenance and operational characteristics of aircraft. At the time of TWA 800, the 747 had tens of thousands of operational flight hours. TWA 800 cannot have been the first time that the 747 had fuel vapors in the center tank that were exposed to the fuel pump. It just doesn’t add up.