When asked why Paul was not invited to take part, Bob explained: ''Because Macca's always done stuff. He was in the band in 2004 - him, Radiohead and Damon.''
He joked: ''But look, on this we've got from the newest to the hippest to the oldest... and he hasn't made the cut, what can I tell you?''
The track - which recently became the biggest-selling downloaded single of 2014 - is raising money to help fight the Ebola crisis in West Africa.
The 63-year-old philanthropist and former Boomtown Rats singer has revealed that he wanted to give the younger stars a chance.
He said: ''The thing is, there's only so many lines, so what do you do? At one point Midge Ure was like, 'We can't take more', so you try and make it the girls and the boys, that's the first thing.
''Then it's who's happening in America - that's really important as sales are quantifiably more over there. Ellie Goulding, Ed Sheeran, One Direction, Coldplay, Sam Smith, U2... all these people are caning it in America.''
Meanwhile, Bob has hit back at critics who branded the track ''smug'' and ''hypocritical''.
He revealed that many stars gave both their time and money to help fundraise, adding to the Daily Mirror newspaper: ''Should we take a pop at everybody who is immensely talented and has money?''
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