I think that there are actually a few reasons, and none of them relate to the skill of the producers in delivering the show.
First, a fair proportion of people are interested in watching real people dealing with situations. It's interesting to see the interaction and how relationships ebb and flow. If it wasn't for this character trait nobody would be watching the show at all. Even if the producers did nothing other other than provide the housemates with food and drink people would watch, especially for the nomination process and evictions.
Second, this series is building on foundations laid by the previous two series. This means that it should realistically hope to pick up the previous series' audience, and then add some more curious viewers on top. This growth is ultimately capped at the number of people from point one. I don't think it's at that ceiling yet, and I do think that had the producers done better they could have had even higher numbers, as some growth was inevitable.
Third, other media such as the internet, SMS and forums etc., mean that BB communities grow and these groups create their own dynamics and their own hype which means you have to watch the show to retain your 'membership'. Plus they mean it's easier to become involved and stay involved which is essential for a show like BB.
BB is an event more than it is a show, now. No matter what the producers do, so long as the housemates are locked in together with their nominations and evictions the show's gonna run. Unless somehow someway it drops out of contemporary culture... it stops being cool... which could happen, you never know....
anyway...