Most of the time, it's because of excessive dirt (and perhaps salt spray, which is highly conductive when moist) in the engine compartment.

To test this, jus take a look under the hood when it's dark, and have someone try starting it. Chances are, you'll see a spectacular laser show! The dirt, when moist, become conductive and drains away the voltage from the secondary (high tension) wires, so there's insufficient spark to start the engine.

See, there was a reason why your dad always told you to keep your engine compartment clean :-).

Well, I said "most of the time..." above. The remainder of instances is most likely that your spark plug wires are probably just too old, and they're cracked and (like with the dirt) just letting voltage get away through other paths. If a thorough cleaning or washing (and then thorough drying, before trying to start it) doesn't help, your next best thing is to change the wires.