Why were they unable to cause the fuel tank to explode when attempting to prove their theory about flight 800?

Because it is impossible to create even a fire much less an explosive mixture inside of a gas tank where the atmosphere is saturatd with hydrocarbon vapors. And that is why nobody has ever witness a fuel tank explosion from a plane, automobile or race car. Or for that matter a molotoff coctail witch is like a small gas tank. When the gl breaks the vapors mix with atmospheric oxygen and catch fire as soon as they are able to burn they can't and don't wait for a explosive mixture.