Programming is a very brain-intensive task. You have to hold all sorts of disparate information in your head at once and synthesize it into extremely precise code. It requires intense amounts of focus. Distractions and interruptions are death to the productivity of a programmer. And an open-floor plan encourages distractions like nothing else.
First of all, you're in a room with dozens and dozens of other people. That's naturally going to be very noisy. People are talking all over the room. The person next to you is chewing on potato chips. You constantly hear people getting up and walking around. Hopefully you at least have carpeted floors, or else it's going to be REALLY loud. All day long doors to conference rooms are opening and closing. Noise breaks concentration. Broken concentration breaks productivity. If you're lucky you're one of those people who can work by drowning out the noise with music through your headphones. Otherwise, you're out of luck.