Canned chickpeas do not work as well for falafel as they have already been cooked and their starches have been released. This makes the falafel mixture too wet and sticky, and it will not hold its shape when cooked, in fact, it will be mushy. On the other hand, dried chickpeas, have not been cooked yet, so their starches are still intact when they are rehydrated. This makes the falafel mixture moist and grainy, giving it the right texture.