Why is diesel, a refined product of crude oil, not immediately flammable but this crude oil was?
Diesel has some of the lighter components removed that would exist in the crude oil. The crude oil will also contain heavier components as well.
The light molecules will flash easier and also typically ignite easier.
Now don’t take the hysterics of the language used in this article to represent fact of flammability.
Also if the train derailment involved tearing up of some of the metal, there likely are very hot spots initially that would ignite diesel or other products as well.