Claire McCaskill explained why very well on MSNBC;
Engel treated him as a whistleblower and only went public when it was clear Bolton was going to reveal all. Did so partially in answer to Trump's tweet in which he asserted Bolton had been silent about Ukraine concerns.

— DianeG 💙🌊😷🏳️‍🌈🧩💉💉💉💉 (@dgPickles) January 29, 2020