Actually, Fudge never believed Voldemort was dead. > “But what do you think he's broken out to do?” said Madam Rosmerta. “Good gracious, Minister, he isn't trying to rejoin You-Know-Who, is he?” “I daresay that is his – er – eventual plan,” said F... read more
Because Fudge was aware he had not done enough to confirm Voldemort’s demise and was afraid the backlash would lead to the end of his tenure as Minister of Magic. read more
Cornelius Fudge and other Ministry officials: The Ministry of Magic never wanted to believe Voldemort was back, and because of their influence on the Wizarding World, nobody wanted to anger the M.O.M. because of its power. read more
Fudge believed that Harry was either lying or insane. To be fair to him, Voldemort was widely believed to be dead, simple as that, and as he caused so much pain before, a lot of people including Fudge would find it even harder to believe he was back simply out of fear that it was true. This was explained well by Lupin in the 5th movie. read more