Short answer: short-term amoral bottom-line mindset. Zucker was a Trump sucker this election, along with Les Moonves among others, because they did not put journalism first or even second.
If anything can be said in their defense, it’s that their sloth may have been an even bigger factor than their cynicism. Trump learned the rules of reality TV so well that he did almost all the work of framing for them. The broadcast and cable shows hardly had to invest any resources in actual journalism. Giving Trump the amount of unfiltered air time they did was resistable, but only if media elites possessed more sense of civic obligation than they actually do.