Why was the Fugitive Slave Act unpopular in the North?
It obligated Northerners to directly participate in slavery, an institution they disapproved of.
An ugly secret of American history is that early police departments in most cities were founded--one city at a time-- not to "protect and serve" and nab bank robbers and jaywalkers, but mainly for the purpose of apprehending runaway slaves. Previously, this duty had been fulfilled by citizen's militias (cryptically referenced in the 2nd Amendment), and this was likewise their primary duty. Most notable crime in the earliest days of the republic was committed by runaway slaves; the act of running away was a crime in itself, and it's not like they had the option of buying food along the way.
Slavery had existed in the North on a much smaller scale, but died out after the revolution. Abolitionism, which regarded slavery as immoral, gained a lot of traction in the North. A tycoon with one or two house slaves had less to lose from embracing this popular new trend than a Southern planter with much of his personal wealth invested in hundreds of slaves.
When slaves ran away, it was usually to the North. Northerners were bound by law to return the runaways to their state of servitude, but were increasingly loath to do so as they came to associate the practice of slavery with immorality.