Why did the Underground Railroad run all the way to Canada and not simply stop in the No?

The Underground Railroad went to north to freedom. When the passengers reaching the free state such as Pennsylvania, Ohio, or New Jersey are sometimes stopped.

Most of the escaping slaves traveled all the way bound to Canada after 1850. The reason why they are escaping it that the Congress in the United States passed a law called The Fugitive Slave Act. And aside from that their safety and freedom have no assurance until they entered the city of Canada.