DigiNotar got revoked partly because they got hacked, but mostly because they had zero acceptable followup. There was no good enough postmortem describing their mistakes and how they would correct them, and they also engaged in a coverup.
Browser makers do not engage in instant execution of any CA that makes a mistake, because CA's are run by humans and to err is human. What they insist on is coming clean when mistakes are made, and steps taken ASAP to restore confidence, like publishing a real post mortem and making business changes to ensure the same mistake can't happen again.
