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Old WWII era vehicles were designed around the extremely muddy fields in Europe. Back then, rubber compounds sucked, and the best way was tall skinny tires that dug deep.
Nowadays tires can be safely deflated to lower PSI without rolling off the wheel, and provide more grip with advanced tread design and wiiide tires.
Having more tires/ extra axles is for safely hauling heavy loads. A goven tire-wheel-hub-axle-control arm-spring-shock and section of chassis can only support so much weight. Adding an axle provides greater payload. Like wing area on aircraft.
DOT has safety regulations, and manufacturers dont want to overrate their equipment, and have a truck crash cause it couldn't stop on a steep decline on a wet rainy road , and be responsible for lawsuits.
