There are many reasons for this. Most people used to be afraid of wolves and thought they were dangerous to humans. For a long time, people tried to kill all the wolves. They would get money for bringing in a dead wolf. This is called bounty hunting.
Poisoning is probably the main reason there are so few wolves left today.
Another reason has to do with the growing human population. People need land in order to live and raise their families. Wolves need lots of land, away from humans, to live and raise their families. As our population has grown, the amount of wilderness where wolves can live has gotten smaller. To help protect the wolf, we will need to help protect the wilderness that is left.