Why do front car hubcaps get dirtier than the rear?

This premise is not necessarily accurate. Car hubcaps generally get dirty from dust, mud, and road grime that is kicked up by the tires. In most vehicles, the front wheels are responsible for more steering and turning, which can cause the tires to kick up more dirt and debris than the rear wheels. Additionally, the airflow around the front of a car can be more turbulent, which can also contribute to more dirt accumulation on the front hubcaps.