I would suggest three reasons. First the two cities vie for the Northern capital status. Two Leeds were dominant in a period when United had a decline. Three there is the whole War of the Roses, York v Lancaster thing. read more
Up until Manchester United were relegated in 1974, as Leeds enjoyed the upper hand on the pitch with only three defeats in 25 games, trouble between the supporters escalated; it then took a more vicious turn still after the Reds' year of mayhem in the Second Division. read more
The teams have met only twice since then, and while polling shows Leeds fans still consider Manchester United to be their main rivals, Manchester United fans consider Liverpool to be their main rivals, followed by Manchester City, Chelsea and Arsenal. read more