Gasoline burns accidentally when fumes are released, as the stoichiometric mixture has to be pretty specific to combust.

Gasoline in a gas tank does not achieve this mixture. That's the entire job of the fuel pump and throttle in modern cars. As the other user said, there are lots of sparks and live electricity in a car crash, it's just not easy to set gasoline on fire or make it explode.

Diesel does not appear to achieve this vapor mixture readily at standard temp and pressure, like gasoline does, and therefore is technically safer in this specific regard.