I don’t think anyone would argue that very few people in the north wanted to go to war to end slavery. Hardcore abolitionist numbers are hard to come by because there was no polling then. Numbers I’ve seen vary from 2%-10% based on memberships in abolitionist society’s.
There was however a very large percentage of northerners who saw slavery as wrong and wanted to see it ended. The Republican Party was founded in part on that proposition. However they didn’t want to go to war to end it. They wanted a return to the founders original intent and put it on the road to extinction.
Even this was to much for the slaveocracy. They didn’t just want to stop it from being out on the road to extinction, they wanted the world to acknowledge that it was a positive good. So they gambled it would be better protected in their own nation. They then lost it all.
Again, the thing that gets me is that there weren’t more abolitionist in America. In a supposedly “Christian nation” founded on the ideal of all men created equal it should have been 80-90% of the people wanting to end slavery by all means necessary.
Well... that's a Lost Cause talking point, but it's just not accurate.
The real truth is that 100% of Northerners wanted slavery abolished in their own states, and a solid majority wanted it abolished in Western territories.
But as for abolition in Southern states, 100% of Northerners understood there was no way to do that, period.
For decades, the Southern slave-power had threatened secession for even discussing abolition and most Northerners took them seriously.
So, for sake of Union, Northerners were still willing to tolerate slavery in the South, in hopes it might be eventually abolished just as it had been in their own states.
Bottom line: it's just not correct to say that Northerners didn't care about slavery, they did in their own states.
But in the South, Northerners cared more about their Union and wanted to see it preserved.