Because the evidence from the wreckage pointed to an explosion within the tank, the only explanation of which is the ignition of fuel vapour. It’s all there in the NTSB report (big PDF!)
Especially under hypnosis.
It is easier to think that a big thing (a missle) caused the plane to crash, than an infinitesimal, virtually undetectable crack in the fuel tank.
I guess that’s true for some people, cap, but I’ve said before that I’ve rarely heard a conspiracy theory I didn’t like and even I have no trouble believing that the insulation on some thirty-year-old wires cracked and an arc was created, igniting the explosive atmosphere inside a tank with only a few gallons of Jet A sloshing around in it.
Which part don’t you believe: that wires can make sparks, or that sparks can ignite jet fuel?
He said he does believe it. Even him.
Yup, even me. And sometimes I’ve believed as many as six impossible things before breakfast. But a spark igniting an explosive atmosphere (some more fuel in the tank would’ve left it too rich to burn) has it all over the other theories.
Oops, sorry - funny how my missing a tiny word like “no” completely changes the meaning of a sentence. 
I talked with ADA (air-defense artillery; “duck hunters”) types when I was in the Army.
A Stinger is a heat-seeker, and the four hottest objects on that plane are the engines, so the Stinger would home in and detonate behind them. The Stinger rarely “strikes” a target, but relies on close-proximity explosion and shrapnel to damage/destroy its target. But I have been told that it does have a terminal ballistic mode that simply turns it into a big bullet, which still wouldn’t account for an explosion in the central fuel tank, just a rupture of the fuselage, possibly causing negative flight control and terminal descent.
It could conceivably rupture the wing fuel tanks, spilling jet fuel directly on hot, even burning engines, and thus causing an explosion in the wing tanks, (still, not a pretty scenario when talking about passenger jets), which could cause sympathetic detonation of the vapor-filled central tank.
So is it possible? From my layman’s understanding, yes.
Probable? I doubt it.
Neither site says anything about a terminal ballistic mode, so that may have been some “duck hunter” talking out his ass. Or it may be information not included in the sites.
I always think of one more thing to say after hitting Submit.
I’m not trying to make a case for the conspiricists; I’m presenting fact and some informed speculation, so that others may decide on their own.
I personally think the conspiricists are nutty as a can of Planter’s.