Cruiser bikes tend to be constructed for the mass consumer market. Even if the price of the bike isn't cheap, the components often are.
* The big balloon tires that are put on them tend to have soft rubber compounds that allow for a soft ride -- but quickly wear away. The white or cream sidewalls that are popular on cruisers don't have UV protection in their rubber compounds so they also tend to deteriorate quickly.
* If you have a rear coaster brake (which tend to modulate poorly), you have to be careful as it's easy to drag/skid the rear wheel and wear down the rear tire down considerably.
You can get a better wearing tire when you replace it. Harder rubber will be longer wearing but will provide for a harsher ride, all things else being equal.