Not sure...but based on my own recent experience, I think it's because the sensation of your body slackening and sinking as you fall asleep is amplified when you're on your back, and it's the memory of having fallen asleep that allows you to notice you're dreaming once you start dreaming. You can feel gravity pulling your body down as your muscles become paralyzed and your sense of orientation goes away. On your side, with a much smaller body area being pulled against something other than the cells in your own body, which you don't feel, it might be harder to get that memorable sinking feeling that alerts you to the fact that you're a minute or two away from Dreamland.