Because the fuel squirting onto the hot manifold was below the auto ignition temperature and petrol lacks the abilty to sustain self combustion unlike diesel which is a totally different fuel. It's all to do with the free / lighter hydrocarbons escaping from the surface, (that's why petrol goes off and when left standing you get that rusty coloured sludge in you tank / carbs, it's the petrol without the lighter / free hydrocarbon molecules innit) even I'm boring myself now.
Also, due to it's below the auto ignition temp, you need a spark or naked flame to ignite the fuel.
That's why petrol engines have spark plugs and deisel's don't.
You can quite easily see why fuels are made conductive for the reason in the video.
Only the fuel vapour escaping just around the nozzle was between the LEL and UEL, hence for the localised flare.
Apologies for boring you all again