Socialist only by American standards (and, possibly, by the standards of certain Tories who still think of the US as a former British colony). By European standards, the Guardian is a fairly typical center-left newspaper. If you want to look at an actual socialist newspaper in the UK, try the Morning Star [1].

The relative popularity of the Sun and the Daily Mail is best explained by Jim Hacker and friends: http://www.youtube.com/watch?v=DGscoaUWW2M&t=1m0s

In other words, a classical broadsheet vs. tabloid situation. Politics doesn't sell as well as gossip and T&A.

[1] http://www.morningstaronline.co.uk/

Ach, come on, the Guardian's incredibly left-wing. Or at least was until recently (less than 5 years).

When I used to buy papers a few times a week, I rotated the Guardian, The Independent and the Telegraph.

If you went outside the news articles the Guardian was foaming at its mouth, agenda crazy leftie. The editors, opinions and letters pages were filled with rubbish. Far more than the Telegraph was politely right wing.

Perhaps they've toned it down since focusing online, but though I don't read the site I still come across op pieces linked in various places that show otherwise.

The Guardian is a broadsheet, not a tabloid. If you want compare it something on the right use the Times or the Telegraph.

The tabloid equivalent of a left newspaper (though the terms make much less sense since the tabloids aren't nearly as political) would be the Daily Mirror, and it more than holds its own in the circulation wars.

They're not exactly socialist, they're centre left. They just look socialist in comparison to stuff like the Telegraph.