What things were rationed during World War 2 in America and why were these things so hard to obtain?

Rubber (tires) it was being used in military equipment.

Gasoline, same thing. Tanks, airplanes, jeeps... all of the military equipment was using gasoline.

Silk, formerly used in ladies stockings, was being used for parachutes.

Sugar, was being sent to the military in packaged foods.

New automobiles were not being built during the peak of the war. Entire production lines had converted to build military equipment.

People were generally willing to do without in an effort to send more to the military.

Many things were rationed, food, water, weapons, etc.