Resources.
Unlike rural communities, urban communities do not always have the right resources to grow thir own food.
Firstly, urban communities are very busy and have lots of buildings packed together. This means that there is very little free space to start a community garden, and these areas could be hard to access.
The is very little 'green space' in urban areas, so to create a community garden you would have to build suitable growing spaces. This could be quite expensive to set up!
The climate in urban areas may not be suitable for plants to grow, and there may not be easy access to water (which we know plants need).