Because it takes evidence. Think about it. Back when Hillary was using private emails, so was everyone else. Trump made a big deal out of it because he was never a politician. So you couldn't prove he'd do it too, which he and his team did. So Trump went after Hillary for something that later on, he did himself. Classified documents case.
Let's not forget Trump made what Hillary did after she did it, a felony, then he did what she did. So sorry that Mueller didn't have enough to charge her. Just enough to make her look bad.
Trump threatened Hillary at their debates that she'd be in jail if he were in charge.
Republicans have been "going after" or "investigating the Clinton's" for years. Obama too. Biden too. Haven't found much. Trump, we have found plenty of crimes and enough evidence to convict.