You dont cancel a flight because of a thunderstorm !!, this can not be the reason.
Of course you can cancel a flight because of a thunderstorm... planes fly around thunderstorms, so it doesn't make much sense to take off into one... taking from my knowledge of meteorology (which was recently expanded by leaps and bounds thanks to a class) the wind shear associated with thunderstorms makes it extremely dangerous to fly in one, do you remember the DL incident at DFW (they made that movie about it, Fire & Rain i think the title was).
It could have been a mechanical, or even a mixture of several factors, but flights can be cancelled in the event of a thunderstorm that causes conditions that are unsafe.
Head in the clouds... yet feet planted firmly on the ground.