Tire Tread Worn out Very Fast
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.