It's because he's a very vocal climate change denier, Chair of The Spectator, which has done a lot to bring some pretty extreme right wing ideas into the mainstream, has a history of boosting Holocaust deniers, has connections with very dodgy Hungarian (and British) right wing think tanks, and is a general all round ultra-Thatcherite roaster. The real question is why centrist types are willing to overlook all this. But then that isn't really a mystery either: they enjoy political theatre, especially of the macho bull___t variety, and look up to male authority figures.
Is his latest pompous stunt a break with the script? It's possible that his ego has driven him to make this gesture of defiance. But a gesture is what it is, and neither the BBC nor CCHQ will be troubled by it: the consensus seems to be that it's an acceptable substitute for the interview itself, so really it gets quite a lot of people off the hook, and all it's cost them is a Twitter-fart that will go unnoticed by most and will be forgotten this time tomorrow.