Hi, I'm pretty sure that is because for many slaves, Canada represented a dream of freedom where slave catchers and lynch mobs couldn’t hurt them. Slaves on the Underground Railroad endured months, and even years, of living like fugitives while bounty hunters and racist government policies were always trying to impede their flight to freedom.
Most slaves started out their journey on the Underground Railroad (which wasn’t an actual railroad but more of a resistance and escape route that was heavily organized by concerned American citizens) by running away from their plantation in the middle of the night. Often the runaway slave was alone, but on many occasions whole families would escape together.
Hope it helps you.