The problem is that most politicians have some kind of skeleton in their closet. Even Trump
For example: Trump's wife may have worked illegally in the US, he may not be a billionaire, .. etc. If Trump goes after Hillary, then it may open a can of worms in the future. It will become acceptable for presidents to misuse their power to "fix" their opposition.
After Trump steps down as President the next president may be a Democrat who may decide to go after Trump. It will become a "tit for tat" kind of thing.