Why did northern states oppose slavery in the United States? Was it simply for a moral purpose or were there economic/political aspects of it?

There were definitely political and economic aspects to antislavery politics. I would go so far as to say the majority of opposition to slavery (or at least slavery's expansion) was political and economic in nature. Bona fide abolitionists--properly defined in the 19th century as people who wanted not only the end of slavery, but enfranchisement of former slaves, were a small minority of Northerners (and were often disliked even by their Northern brethren). If you were to poll Northern residents about the morality of slavery, most would probably say it was "wrong," but they could shrug it off easily enough.

Almost from the beginning of the Constitution, there was considerable political resentment at the South's outsized influence due to the 3/5 clause concerning representation. It was so contentious that it featured prominently in some Antifederalist literature. This problem was amplified by the Senate, since all States received 2 Senators regardless of size, and as western States grew, Northern population considerably outstripped Southern population by the 1830s, yet the South's voice in the Senate was equal to that of the rest of the country. To add to that, the political class of the South was connected to slavery in a much more intimate way than Northern politicians were connected to the growing industries and financial interests in the North. Then as now, money talked in politics, but in the South the planter class tended to be the political class, such that almost all Southern Senators, governors, and other high office holders were themselves slaveholders, many with enormous estates and a vested financial interest in the slave system. Thus a considerable number of Northerners came to resent and fear what was termed "The Slave Power" (or sometimes the Slave Power Conspiracy).

The Mexican War provides a good case study of this. Although many Northerners shared a desire for expansionism (and often shared racist rationales of Southern expansionists), Northern Whigs were the least likely members of Congress to vote in favor of the Mexican War, castigating it as the effort of an aggressive Democratic Party seeking to serve the interests of its aristocratic, Southern masters. Abraham Lincoln, in his only term in the House, took to the floor to condemn President Polk and the war faction largely for this reason. The thinking went that most of the new lands captured within Mexico would be opened to slavery, and provide the South with even more political clout. Pennsylvania Democrat David Wilmot attempted to push through a provision that would have outlawed slavery in ceded territories, but it was easily defeated in the Senate.

This gets us to the core of antislavery economics--The Free Soil Movement that blossomed after the Mexican War. A considerable number of Northern workers and farmers feared that if slavery were to expand into western territories unchecked, it would crowd them out. Small farmers believed (rightly) they could not compete with the economic power of plantation holders, and pro-slavery interests bought up many of the richest lands near the Mississippi and Missouri rivers in Missouri, and were making inroads along the Kansas River in Kansas as well. Free laborers also disdained the presence of slavery since it in essence drove the wage floor down to zero (a number of these "Free Soil" men had no great love for slaves, either, despising them alongside their masters). The precedents of the Kansas-Nebraska Act and the Dred Scott Case were not lost on Northern politicians, either, since it was now theoretically possible for industrialists to purchase slaves in Kentucky or Missouri, drive them across the border to the North, and set up a factory staffed by slave laborers, as Lincoln alluded to in the Senate campaign of 1858. Even where farming was not a major concern, free laborers feared the possibility that in western, mountainous states, mining operations would be conducted by slaves if they did not block the ambitions of the slaveholding class.

Summary:

Northerners resented the outsized role that slavery gave the South in terms of representation.
Farmers feared being crowded out by plantation holders and dominated by a slaveholding aristocracy.
Free laborers disdained the further curtailing of their rights and economic fortunes through competition with slave labor.