It looks like the only options they checked were a control group who sat all the time, and experimental groups who walked for different durations. I can see some advantages to doing this. Using just one form of exercise makes the experiment simpler. Also, everyone already knows how to walk, and you don't have to be in very good shape to do it. If you have someone who's used to a totally sedentary life attempt 30 burpees at the top of every hour, there's a decent chance you'll put them in the hospital.