Because Maestro is the largest banking format used across all of Europe. It doesn't accept it, nor does it accept it's little sister, Solo.
It forces people to use Credit Cards or bank branch Visa's, of which HSBC, NatWest, nor Lloyds use, it's the smaller branches like Barclays and Nationwide who do. As a result I cannot buy on Xbox360's service because they've isolated a massive section of home users who have the largest card format used in Britain and widely used across Europe.
Sony give the option to use Maestro, Solo, and all credit cards.