According to the show's official BBC site:

"Following his unexpected departure from the show, announced today, John Sergeant has issued a press statement. John said he was sorry to say he had decided to leave Strictly Come Dancing, adding; 'It was always my intention to have fun on the show and I was hoping to stay in as long as possible. The trouble is that there is now a real danger that I might win the competition. Even for me that would be a joke too far. I would like to thank Kristina and all those viewers who have been rooting for me through the series.'  In response, Jay Hunt, the Controller with BBC One said; 'John has told us of his plans to leave Strictly Come Dancing and we are very sad to see him go. He has been an entertaining contestant and is hugely popular with the viewers. We would have liked him to stay but we respect his decision to leave.'"

Naturally the BBC would've liked him to stay - they're going to lose a wedge on lost phone calls now. Sergeant got a measly score from the judges last week and yet still didn't end up in the final two. That's some people power right there... power which has now been snatched away. The people will not be happy. They're already spending a small fortune phoning for their favourites on ITV's X Factor and I'm a Celebrity...
 
In fairness, it's not the Beeb's fault John skidaddled. It's Cherie Lunghi's. The 56-year-old contestant, who John felt compelled to apologise to after she got kicked off the show last Sunday, said: "Kristina (John's dance partner) is a wonderful dancer and makes him look wonderful. I don't know whether it's a Marilyn Monroe fantasy or whatever, but it's a show and it is entertainment. It is almost like there are two shows going on. John is dancing and competing, but there is this whole soap thing and sympathy for the character."

There's also a load of aged sitting at home with nothing but the box and their phone for company. They're most likely organising a march as we speak, complete with effigies of the judges' heads on sticks.