[mikeash]

Really old cars have very simple electronics. Even after computers started showing up in cars, they were pretty simple and didn't interact much with the security aspect of things. When you start a car like this, you're just making a connection between two wires to power up the electronics, and briefly making a connection between another two wires to run the starter motor. The only security in the whole system is provided by the fact that the connection is made by a switch that requires a key to turn it. If you don't have the right key, you can't turn the switch, and that means you can't connect the wires.

The trouble is that the wires must be fairly exposed to the occupants of the car, since the switch has to be accessible. That means you can just bypass the switch entirely by removing the appropriate covers and attacking the wires directly. This is "hotwiring."

Physical locks are also not all that difficult to defeat directly. You can pick an ignition switch much like you might pick any other lock.

Starting around the late 90s or so, car manufacturers started adding more robust security measures. These include things simple like locking the steering column when the ignition switch is off (thus preventing you from driving the car after hotwiring it), all the way up to authenticating the key with a relatively sophisticated protocol, and having the engine computer refuse to run the car unless it can sense a real key.

As a result of these changes, the list of most stolen car models is still topped by cars manufactured in the late 90s. Low-end Hondas from around 1998 are right at the top of the list, because they occupy a sweet spot of being relatively valuable and still fairly easy to steal. Modern cars are stolen literally orders of magnitude less frequently; about 100,000 older Hondas stolen per year in the US, whereas new cars are stolen at a rate of hundreds per model per year at worst. Also as a natural result of these changes, car theft is way down in the US. About 700,000 cars were stolen in the US in 2013, compared to almost 1.7 million in 1991. Pretty much the only way to steal a newer car is to either tow it away or steal the owner's keys. (A common scenario for car thefts is a burglary turned into auto theft when the burglars find car keys in the house.)