Cause Hugo Chavez had terminal cancer, Mandela was already 95 and very frail in his last years, while George H W Bush had bouts with less serious diseases and vascular parkinsonism, which is an incurable but slower (chronic) disease.
That being said, he is still frail and I don't think that he has too many years left in him. (But then again, I thought the same about Zsa Zsa and Billy Graham 5 years ago.)