Why should the Indians saved the buffalo instead of killing every single one?

I'm sorry, I do not know what text you are using in your history class, but the Native Americans only hunted the number of buffalo their tribe needed to survive. The Native American peoples used every part of the buffalo and left no waste. Buffalo hides served as both clothing and integral parts of the teepee. Untanned hides were used for making foot wear (moccasins). The buffalo was revered, not slaughtered without reason.

Buffalo meat was dried to preserve it; organs and the tongue of the buffalo were considered delicacies. The hair of the buffalo lined clothing for warmth and served as the basis for making rope. Bones were used to craft tools, weapons and toys. Horns became drinking gourds. Buffalo teeth were even saved for use making rattles and jewelry.

The Native Americans depended upon the buffalo for survival. They did not kill more than they could use or carry. Nomadic people, the need to move preempted acquiring anything that could not be used.