Canned chickpeas just don't work as well. They are wetter and mushier, so your homemade falafel is more likely to fall apart as it cooks. Additionally, they have a pastier texture, while traditional falafel should be light and fluffy inside. Some people also find that canned beans have a metallic aftertaste, although I haven't noticed that. Overall if all you have is canned chickpeas, you can use them, but they won't be as easy to make or taste as good.