He's about to turn 79, he visibly touts an unhealthy diet and lifestyle, and his dementia (that malady seems to have secondary health implications) seems to be worsening.
He's also pissing off a lot of powerful people who, depending on which faction they're part of, may not always get to choose who becomes president, but who are pretty good at deciding who doesn't get to remain president, and not necessarily just at election time.