People have short memories.
When Boris resigned his Henley parliamentary seat to become London Mayor, there was rejoicing in the Conservative Party leadership. He was seen as a gaffe machine, off-message and a loose cannon. Cameron thought he was a liability at Westminster They could not see the back of him quick enough.
So what has changed?
Eight years later he is now seen as Cameron's natural successor. Conservative leaders like to influence whoever follows them (Thatcher chose Major) and Cameron does not want to be ousted by Johnson. This has nothing to do with Boris' celebrity or political standing, it is about keeping him from ousting Cameron. If it were about celebrity why didn't they bring him back in 2010 when they had a chance of a majority?
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This is how Boris' candidature for Mayor was reported in 2008...
// The Conservatives will hope that the flamboyant, gaffe-prone Henley MP will be able to break Labour's hold on the position.
Mr Johnson has garnered headlines as often for his behaviour as his politics.
Comments about Portsmouth and Liverpool, allegations of infidelity, and rip-roaring appearances on Have I Got News For You have added to a larger-than-life public persona that will ensure an entertaining, and potentially tight, Mayoral contest. //
http://news.sky.com/story/574491/boris-johnson-wins-tories-nomination-for-mayor-of-london