Because 6 is more than 4

1976 Tyrrell P34 6 wheeler. production of the car was ceased when Goodyear refused to produce the 10" front tires since only 1 team used them. Today, the current owner Simon Bull runs the car since Avon Tires agreed to produce the tires. This allows the car to be seen at historical events today

The concept behind the design was to reduce the drag caused by larger front tires and produce a cleaner airflow over the chassis.