Chicken is cheap because the feed conversion ratio is very good: apx 1.6 kg of feed (corn) in yields you 1kg of chicken out. Only some varieties of fish does better. In comparison, beef has a ratio of around 5 to 7 (10 if you just count the meat).
It's also cheap because there are very few protections and regulations for the animals and the fact that selective breeding has gotten their farmed lifespan down to just 6 weeks.