Book titles: How a Colon Makes a Best Seller

Just browsed through The Economist's list of their recommended books of the year. Out of about 50 books more than 30 have a colon in their title, e.g, Wikinomics: How Mass Collaboration Changes Everything. Why? How come all non-fiction books nowadays always seem to have a colon in their title? Are there no more good book titles without a ':' left? Is this like the web 2.0 companies that are called things like spinr, coolr etc because there are no other domain names left? I don't get it and I don't like it (I guess this means I'm really getting old)!