Because it's known for it Con - McAfee/Norton's are the 'industry leaders' in virus/security here in the UK and gets sold discounted or on 30 day free trials in bigger PC outlets like PCWorld. It significantly slows down or blocks download speeds on literally everything coming into your PC including M/S updates and the likes. Usually I don't even bother setting it up whenever I upgrade kit with it on as standard, without a choice as to whether you want it or not.
AVAST or AVG have free versions that are as good as (or better than) Norton and don't tie up valuable RAM, so are much faster and their paid versions are cheaper and superior in features for the average home/gaming use. McAfee/Norton are really too heavy duty/high spec for the average user and sell more for corporate and commercial database use, where they need more intense firewalls etc but they're greedy enough to want a share of the home market as well, hence the slightly reduced but still premium prices.