They are successful largely because they were allowed to run a monopoly on satellite TV in the late 80s and build up a very dominant position, which they still hold onto.
Though not on sports, they'd argue that the BBC have a monopoly, and in many ways they'd be right. You immediately say monopoly and people think badly of them but I really do think their product offering is good value for money (if you like football). Even though they've dominated the market it's not to say they haven't still innovated - with Sky+, then HD, anytime phone & broadband services, more and more channels, smaller and less conspicuous satellite dishes, online tv on demand. Admittedly I used to work for Sky, but I do believe they provide an excellent service.