Reagan was falling apart at that point, he's out in spite of a ridiculously fortunate tenure (for him).
Eisenhower was also getting too old, and he was reluctantly drafted in the first place.
Bush and Obama both are simply too hated and locked up by the polarization to want to do more.
This leaves it to Clinton, still young and powerful, and still very popular with the job.
This, except that I think Obama would have gone for a third term and he is an insanely good campaigner, so he might have made it despite all odds.
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