Deliberately mismanaging a company would be a violation of the executive’s fiduciary responsibility.

The problem is that you’re moving from a simple easily-proven fact to something which requires two judgement calls. Any prosecutor can show that you made a trade before certain information became public, but proving that something was mismanagement (as opposed to a mistake or bad luck) or that it was a deliberate attempt to play share price games is so much harder that most executives would evade punishment even if pretty much everyone would find their actions hard to explain otherwise. White collar crimes are already enforced laxly due to the challenge of needing to prove intent in many cases, there’s no benefit to making it even easier to defraud investors.

Well then good thing it was only "accidental."

This also ignores the large number of people with insider information who are not execs at the company. Clients, partnering firms, etc. If it's all about fiduciary duty, then the law firm doing a large M+A deal should mismanage that deal, so they can trade on it for their own profit.

And then would be fired. Getting their golden parachute and insider trading bonus.

Only problem is humans constantly fail to take, or purposefully avoid, responsibility.

Good luck proving it.