IMO because John Lennon was an outspoken individual, mainly about peace, when your hugely famous and you do that it gets you into trouble with the press and Government agencies as they dont agree or dont like what you are doing, MI6 took a strong interest in him because he was speaking out against the troubles and said he was prepared to meet or do anything to help solve the problems and killings, there was allegedly a smear campaign against him by a member of MI5 and certain newspapers saying that he donated funds to support the IRA and do concerts for them. There is absolutely no proof whatsoever that he donated funds to the IRA, Yoko has been asked about this many times and always denied it and the dead cant answer for themselves.
It was the same story when he went to the US and protested against the Vietnam war, Nixons government were worried about the effect he was having on young people's views and so tried to deport him and stop him getting a green card, they even tapped his phone.
He is credited with helping bring an end to Vietnam by instigating mass protests on the streets of America.
A great man IMO, a genius no doubt, flaws ? of course he was no angel but he was prepared to put himself on the line for his beliefs (peace) he is on record doing this on many chat shows like Parkinson and Dick Cavett..
Of course there will always be the odd book with some new 'revelation' about him which will guarantee the author a few thousand sales but overall, as a fan, Im glad he was like he was, I dont remember Elvis putting his neck on the line for anything he believed in.http://www.independent.co.uk/news/uk/this-britain/lennon-donations-to-ireland-did-not-go-to-ira-says-yoko-ono-726461.html