It mattered more to the South.

I'll let Ulysses S. Grant speak on this:

"“I felt like anything rather than rejoicing at the down fall of a foe who had fought so long and valiantly, and had suffered so much for a cause, though that cause was, I believe, one of the worst for which a people ever fought, and one for which there was the least excuse."

That's what Grant wrote in his "Personal Memoirs," published after his death. After the war, the federal government and Northern citizens had physical accomplishments to point to. The Union had been restored. Most of the fighting had taken place on Southern battlefields, and while it had taken a tremendous toll in lives, Northern homes weren't destroyed; Northern homes weren't burned. If anything, the economy was stronger than ever, courtesy of a massive influx of federal spending and investment in infrastructure.

Northern soldiers received pensions that were (for the time) generous, and only grew more so as time went on. They were acclaimed as heroes, paraded and proclaimed, given medals and parties.

The South had none of this. It had been ravaged by the war, and the men who returned were psychologically ─ and in many cases physically ─ damaged. The question arose again and again: "Why did we fight?"

The idea of the Lost Cause, a term coined by Edward Pollard and taken up by such men as Jubal Early and Jefferson Davis, was an answer to this question and a means of addressing the psychological anguish that resulted from it. As Drew Gilpin Faust writes in This Republic of Suffering: "the celebration of Confederate memory that emerged in the ensuing decades were in no small part an effort to affirm that the hundreds of thousands of young Southern lives had not, in fact, been given in vain."

There were alternate narratives, of course. Look at Grant's own "Personal Memoirs" as one example. The emancipator Frederick Douglass saw the development of the Lost Cause myth as a threat and worked hard to fight it. Northern writers wrote just as many things as Southern writers about the cause and reasons for war, but Southern writers cared more. Winning the historiographical battlefield was all they had: They lost the physical one.

If you have the chance, pick up a copy of The Myth of the Lost Cause and Civil War History, which includes nine essays that get exactly to the heart of your question. The last essay, one I find particularly compelling, discusses how the Lost Cause became a religion in the South, adopting many of the aspects of Protestant evangelicalism.

With all that in mind, let's come back to my answer. It wasn't that the North had worse arguments or argued them less; it was that Southern writers and supporters of the Lost Cause idea cared more. Memories of the war faded in the North; the federal government had won, the Union had been preserved. But in the South, the memories stayed fresh. If Southerners let go the idea of the Lost Cause, they'd have to admit that Southern culture was barbaric, that the "peculiar institution" of the South was nothing short of inhumane and worthy of extinction.

Follow up: Is there a general consensus that an extremely sympathetic view of the South dominated historiography until recently? The Civil War is far from my area, but I feel that is a rather bold claim.

It really depends where you're studying it in my opinion. I think the narrative of the 'Lost Cause' and the narrative of States Rights over slavery being the cause is much more of a case heard in the US.

In the UK, the narrative that's told in popular culture is very much of the valiant, abolitionist North defeating the slavers in the South. It's only when you study the thing you realise it's a lot more nuanced than that. In more learned liberal circles, I'd say the general opinion is that the Union were definitely not fighting for abolitionism, but the 'rights' the Confederacy cared about the most seemed to be focused on slavery.

It's more in learned right wing circles that the idea of the Lost Cause has a bit more traction - that's not to say it's inherently a right wing idea.