"Again, only speculation, but perhaps the very high speed near-vertical impact into the hard surface means that the structure of the aircraft and the uncontained fuel is dispersed too rapidly and widely for any flame front to keep up."
Many years ago I read a story about the development of Napalm. They wanted to use an explosive to scatter it far from the impact site. But if they made the explosion too powerful, the speed at which it traveled simply exceeded the flame propagation rate and no fire occurred. I wonder if the crash conditions created conditions necessary to allow this type of to phenomena to occur?