Pressurization also has a lot to do with it. Just about anything is explosive in the right conditions. Especially in dust or vapor form. Anyone who lives in the Midwest has no doubt heard of grain elevator explosions. And you only have to stick your head in an aircraft fuel tank to know fuel vapor exists inside that tank. It is almost always completely safe. So safe, in fact, that the 747 (and many other aircraft) are designed to use their fuel tanks as heat sinks for cooling electrical equipment. The jet I fly is actually one giant fuel tank full of wiring and other plumbing. Part of that design includes insuring there isn't a source that could introduce a spark into that space. But like anything designed by people, mistakes (including design errors) are made. Just because it doesn't happen often, does not mean the problem doesn't exist. It only has to happen once at just the wrong time. Recently, I blew out a hydraulic actuator on a speedbrake that had never failed in the history of F-16 operations. It drained one of my hydraulic systems in a matter of seconds. After I landed, the factory sent out representatives to take pictures and analyze the part that failed. It had never happened before...until that day. But on that day, all the right conditions occurred to cause the failure. No conspiracy, no cover up, no trend item, and at least in this case no disaster. Just a failure of a system that up until that point had never failed.
That was a mechanical failure -- not a chemical explosion that had never happened before.