Before the Civil War, why did wealthy powerful Northerners

[zipplewrath]

2. Economics

That's usually the standard explanation. The north was an emerging industrial/capitalistic economy, and the slave states tended towards agrarian. Slave labor isn't all that desirable in factories and in merchant type economies. Europe also objected to it causing some diplomatic and trade problems with them.

Quite honestly there were many reasons. Some religious, some philosophical, and some were basically just raw power politics.