Because it shows politically motivated use of legal power, which is especially bad. I'm all for prosecuting Trump and Clinton and all the rest of them, but to only prosecute Trump when there's a sea of malfeasance stinking up the place seems like an overwhelmingly bad idea.
The article referred to in the parent asserts that Clinton was also prosecuted (albeit for a different, and lesser charge). As many others have been. So the argument you're making here seems rather weak.
The solution is either to punish them all or none at all? That doesn't help, at all, to bring some kind of justice. Or would it be the case that given it's a sea of malfeasance and it's impossible to prosecute all of them to simply give up on the law and let it be a free-for-all?
