Obviously because it was radar guided to the center of mass (thus enabling the center fuel tank cover story, where that wouldn’t be possible with a heat seeking engine hit). But you made a good point - the missile wasn’t a heat seeker, so that gives info on what it could be - and what was needed to launch it. Thanks.
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According to the NTSB, the very same air conditioning packs below the center tank, which helped make the vapor in the tank flammable by heating, would have been a viable target for a heat seeking missile.
It seems the engines would have been way hotter, though.