Because Clinton has a rabid following.
I understand people like him, but just because you like him doesn't mean you put him as the greatest in history.
Clinton is the most recent expresident. It gives him an advantage. He has a clot of diehard followers sort of like the Saddam dead-enders in Baghdad.
As for the Founders, nobody know very much about them anymore except that they were white male slave-owning fascists.
The increase in President Reagan's vote may be attributable to the generally favorable publicity at his death.