Because God wants us to take care of our bodies. In the Bible he calls our bodies "temples" and that we should care for them. Purposefully harming your body for pleasurable or gluttonous purposes is offensive to God.

Section 89 of the doctrine and covenants reminds me of the commandment given to the Children of Israel, wherein they were commanded to avoid different types of food.  
 
Was it that the food was bad for them that they were forbidden to eat? No, that wasn't it...Then what do you suppose it was about? To me, it was about separating themselves from the world, having faith in God's commandments, and proving that faith through their actions. 
 
I believe it is the same today with God's law of health.