Why is Hillary considered untrustworthy by some?
Clinton has always been the most obvious Democratic nominee. Therefore, she is the biggest threat to the GOP. Therefore, she is the biggest target.
Here's the thing. Clinton is no more corrupt or vile than any other politician. That's the plain, simple truth. Every last one of them is equally untrustworthy.
Whoever is the biggest threat gets the most attention. Whoever gets the most attention makes the most impact on the national psyche. And because of that, the Right does its level best to make sure that attention is as negative as possible.
If Joe Biden has been the assumed nominee all this time, he would be under such heavy attack right now that Clinton would look pure as the driven snow.