It's called a 'post mortem' in the medical world because the patient died. It involves a high level of inspection into what went wrong and what can be learned to prevent it. I assume the term was adopted from there into project management.

It might make a little more sense in the world of shipping software in retail boxes, where products/projects had a 'done' date. The project is dead, what contributed to it's demise? Or you might generalize death into failure, and that's why we use the term instead of post-incident.