Well, people have been investigating Trump for a year or so. There doesn't seem to be anything there. Hillary, on the other hand, is plainly in felony violation of several federal laws, and it could even be argued that Comey could be charged with obstruction of justice over his mishandling of her investigation.
But before Trump could be impeached, wouldn't he have to be guilty of some crime? With what crime do you think he can be charged?