Why did the north wanted to count slaves as property?

They didn't particularly.

It was the Supreme Court, influenced by the elderly Chief Justice, Roger Taney, that interpreted the Constitution in this way. Taney reckoned that when the Founding Fathers had declared that a man's property was sacred, they would have included slaves in their definition of property.

So was the South that applauded this decision - strictly, it meant that slavery was legal in every state of the Union.