The very short story is that unlike with something like gasoline combustion, where the energy output basically happens nearly instantaneously, a nuclear fission event releases energy both directly (from the fissioning) and indirectly (from the subsequent radioactive decay of the fission products).
This indirect energy output is small in comparison to the reactor's output, but a small percentage times a very large power output is still sufficient to meltdown a reactor (and boil away the coolant in the process) if there's no way to remove the heat being generated.
Imagine what you'd have to do with a car engine if every car trip of 4 hours or more produced 4% power output in the form of heat for the next day after your trip was over.
This indirect energy output is small in comparison to the reactor's output, but a small percentage times a very large power output is still sufficient to meltdown a reactor (and boil away the coolant in the process) if there's no way to remove the heat being generated.
Imagine what you'd have to do with a car engine if every car trip of 4 hours or more produced 4% power output in the form of heat for the next day after your trip was over.