Because David Cameron is a viable alternative?
I'm a true believer that Labour have been in power this long because there is no viable alternative. If there was, Blair would have been out of there at the last election.
There has been a common opinion against government whether it is central or local but they have still remained in power.
I think the only real difference these days is that the "protest vote" is no longer aimed at the main opposition party but at the minority parties like UKIP and the Green Party etc.
Oh well, back to looking forward to the unknown!