How do you know some charges weren't set off by the crashes but masked by the huge fireballs created by the combustion of aerosolized jet fuel? Secondly, explosives can be engineered so that heat alone will not detonate them. For example, high explosive RDX requires the simultaneous delivery of high heat and pressure to induce detonation. Thirdly, it is relatively easy to design casings for explosives that would allow them to survive even the most violent assaults. The casings of jetliners' black boxes protect their contents from impact accelerations of 3,400 Gs and from temperatures of 2,000ºF for up to 30 minutes.
You're not even thinking about this rationally are you?