Going back to comments made by a passenger on a link in the OP, they were smelling fuel fumes in the cabin during the flight.
For those who are regularly around aircraft, ask yourself, What would the most likely source of fuel fumes in the cabin be? Now add the background information that you have just had a problem with oil quantity indications and engine vibrations sufficient to pull the engine back to idle.
Maybe this crew didn't know their systems well enough to logic out the likely core problem, but maintenance should have been able to figure out the possibilities.
The core problem of fuel leaking from the nacelle was not suspected by the flight crew, thus they did not look for confirmation, they did not request that the fire trucks be at the ready, and they did not operate the aircraft in a way to minimize the possibility of fire.
As a result, the crew was blind-sided by a fire that they were not expecting. There must have been sufficient glare from the fire that they knew something big was wrong, even if they could not see the wing from the cockpit.
Normalcy bias set this crew up for a big error. Suppose the trucks had been at the fire house instead of out on the airfield proceeding to another event. Suppose in the urgency of the event, some trucks got stuck taking a shortcut. Each fire is different, and one cannot blithely expect that fighting the fire will proceed in a linear fashion. If the fire had breached the cabin, it would have become a high tech gas chamber. Evacuation is the only logical course with a large uncontained fire.
BA got it right at Las Vegas. SIA got lucky.