The Mail is a popular paper because it has a high news content and features that interest a lot of people. Contrary to what some here believe people's choice of newspaper is not always influenced by their political views, many either ignore the papers leanings or take them with a pinch of salt. The Sun's main readership is probably more Labour than Tory despite it's traditional right wing stance, it is bought in the main for celebrity gossip, sport and TV coverage.
Those who bang on about Daily Mail reading scum and Sun reading morons really ought to get out more, their narrow view of how the world works needs educating.