Because of the economics of distribution. Information gets distributed by duplication, whereas physical objects get distributed by transfer. That makes control of distribution of physical objects more plausible, because the number of instances you need control over is limited by physical constraints.

Note: more plausible. You can easily point to examples where physical object distribution control has failed, and where information distribution control has succeeded (Prohibition and video DRM are the examples which most obviously spring to mind), but those cases are the extremes.