Because the wings are where they keep the fuel. Most of us here are not pilots, and it was in my mind that there might possibly be a centre tank on the A320 for transferring fuel or something. What do I know?
Inflight smoke--fire-- causing fuel tanks to ignite?.
Jet fuel is actually quite difficult to ignite. Like diesel. No information re temperature of wing tanks at FL370 after several hours in cruise
OZ214 Asiana crashlanding at SFO. Cartwheeling down the runway, the fuselage burned due to hot leaking engine oil which took 15 minutes to burn the fuselage. The wing and center tanks never erupted.
QF32 Qantas uncontained engine failure - extremely hot turbine fragment tore through one of the wing tanks - fuel did not ignite
B777 BA2276 uncontained engine failure engine fire, winds blew fire onto fuselage. No wing tank explosion
Fuel tank related accidents in the past:
B737-400 TG114 - faulty wiring in centre tank and airconditioning packs below centre tank caused nearly empty tank to heat up and explode on a hot day while aircraft on the ground.
B747-100 TWA800 - wiring sparks causing explosion in centre tank which was heated by aircon packs under the tank. Aircraft delayed departure (causing longer heating of centre tank by aircon packs while on ground) and was climbing when explosion occured.
B737-300 PR143 - damaged wiring in centre tank, aircon packs under centre tank causing heating of centre tank which was empty, explosion while on ground. Hot day
Concorde AF4590. FOB ruptured tyre causing rubber fragments to indirectly rupture wing tank. Fuel ignited by hot engine
Lithium battery:
UPS6 Uncontained cargo pallet fire causing disablement of pilots through smoke. No fuel tank ignition.
A380 does not have centre tank
Not to say it did not happen but just providing a different perspective.
Most commentary are now walking back from inflight explosion as a cause and raisin onboard fire as a possibility however uncertainty and lack of information clouds every theory. one commentator said - "a bit fast for a fire but a bit slow for a bomb"