Why were many Northerners upset with the Compromise of 1850?

They disliked the adoption of a strict fugitive slave law.

The Compromise restricted freedom of slaves in the South.

It seriously affected the labor supply of Northern manufacturers.

The main reason why many Northerners were upset with the Compromise of 1850 is because " They disliked the adoption of a strict fugitive slave law," since this meant that any northerner who did not assist with the capture of a fugitive slave could face penalties.