Why did some white northerners did not want slavery to end?

Because they thought it would threaten the cotton revenues, which were more than 50% of US exports.

People don't like recognising that most Northerners were not Abolitionists. They were fighting for the unity of the states, and by implication, the cotton revenues.

It was only after the Emancipation Proclamation that the war was made to look like a crusade against the evils of slavery - and that was principally to keep Britain and France from granting recognition to the Confederacy.

No doubt a few Northerners did take another look at this thing called slavery, and began to feel a glow of moral mission in the great struggle. But the mid-term elections did not seem to indicate this, and it simply wasn't the biggest factor.