Doing so would be legally controversial and likely reduce the odds of peaceable resolution.

Source: Why Robert Mueller probably won’t — and perhaps shouldn’t — indict Trump – The Washington Post

A sensible case for why Trump should not be indicted whilst President. That does not mean he could not be indicted when he is no longer President. A pragmatic decision not to indict whist 44% of Americans think the Mueller inquiry is a witch hunt may change if the Mueller inquiry reports and presents evidence which starts to change the minds of the 44%. Not an easy task given they are not moved by what comes out of the Presidents mouth every time he opens it but if a smoking gun were found linking Trump to Russia patriotism may overcome loyalty to him.