It's easy to get. Healthy and nutritious food can be more costly, and being poor you have very limited amount of funds. It can take more effort to make and being poor, you might have much less time for yourself and have a physically or mentally pretty crushing job so less energy to prepare food. It might not be as satisfying for the brain as unhealthy food, and being poor thing might suck balls so you might not want to give that up and just want something good in your life that makes you a bit happier, even if it is not great for your body.

Last part is true for beer related weight gain too. And lot of it goes for having active and healthy lifestyle.