Because it's really overblown. The Clintons are, bar none, the most heavily investigated people in American political history and there was never any evidence found of corruption. All the stuff which Republicans howl about are mostly either baseless conspiracy theories, like Uranium One, or stuff that looks maybe bad on its face but doesn't turn out to have any substance behind it.'
At this point it's also really ******* tired. Hillary Clinton lost the election three years ago. The Clintons aren't going to be relevant to public life anymore. So the constant Republican harping on them borders on psychosis. Instead, Republicans should be paying attention to the very real corruption going on with Trump, such as the money which appears to have been embezzled from his inaugural committee, or the people who are bribing him by patronizing his Washington DC hotel.