Urban areas are a lot more polluted than rural areas becuase there is more traffic, factories and rubbish.
Pollution can contaminate soil in urban areas. Toxins and chemicals from peoples rubbish can also contaminate soils. If you grow food in contaminated soil it could be dangerous to eat as we would be ingesting the chemicals into our bodies.
Contaminated soil can also prevent food from growing correctly, so it would be a waste of resources used to build a community garden.