I will answer that you for you.
Because Donald Trump is running for President and he already smoked the establishment once and they are worried he will do it again.
Because New Yorkers hate him which is unavoidable once you get to know him. DAs are politically elected and though district attorneys are considered to be quasi-judicial officers they seek to please their voters politically.
Donny's problem is much of what would have protected him was just norms and not law. He is the grand breaker of norms and others can do the same to him.
He also broadcasted his grift game and people started to copy him so it makes a lot more sense to make an example of him instead of dealing with scores of wannabes.
Now go to bed.