Because it is far more satisfying to slash 3.
Though seriously, slashing all four pretty much renders the car immobile (and also covered by insurance), but slashing just one will leave the car in a more/less driveable state and will also leave a nasty bill for the person who owns it.
So slashing 3/4 will maximize the amount of money they have to pay (and damage done), while not reaching the point where their insurance covers it.

Also, if you slash only one, they are likely to have a replacement tire, so you won't ruin their day.