The US Constitution specifically required that. If the Northern states refused to do it, it was breaking the agreement upon which the Constitution was conditionally ratified.
The section of the US Constitution which specifically required all states to return fugitive slaves is Article IV, Section 2.
You won't like it, but it is in there.
“The section of the US Constitution which specifically required all states to return fugitive slaves is Article IV, Section 2.”
A state is has no obligation, under the Constitution, to return a runaway slave if no claim for that runaway is made by the party to whom such service or labor is due.
Article IV, section 2, clause 3.
No person held to service or labor in one State, under the laws thereof, escaping into another, shall in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up on claim of the party to whom such service or labor may be due.