Is it true that one of the major reasons that the U.S. fought the revolutionary war was to preserve slavery? If so are there any other major reasons or was this really the main one?

Assuming you're asking about the assertion Nikole Hannah-Jones made in the New York Times Magazine's 1619 project, it's true in part but not in whole. There were certainly southern enslavers who joined the Patriot cause as a means to preserve slavery, or because they were worried that the Regular soldiers would arm enslaved people and use them as troops (which they did at times). However, Hannah-Jones probably did overstate the preservation of slavery as a major cause for the Revolution; it's complicated, and reasonable people can disagree over what "really" happened, but even scholarship from when I was in graduate school, in another century, cites fear among enslavers that their enslaved persons would be turned against them as a revolutionary motivation in Virginia and the Carolinas.