Simple: credibility. Right now, the default position is that they're insecure and they don't know what they're doing. A post-mortem with a solid RCA shifts that

Simple: credibility. Right now, the default position is that they're insecure and they don't know what they're doing. A post-mortem with a solid RCA shifts that to "we've identified this, fixed it, and put in place systems to ensure it can't happen again." It's the aviation industry playbook; air travel is perceived as safe (partially) because of the big song and dance they put on about safety analysis after an incident.