There are basically only three reasons why threads disappear or are closed:
1. The OP PMs or emails or calls and asks that their thread be closed. Happens more frequently than you'd imagine.
2. The thread is off-base from the start. Example: "'Bush's War in Iraq Sucks' - Discuss"...
3. The thread is so badly hijacked in such short order or over a longer period that cleaning up the thread would take more time than any moderator is willing to spend going through the thread and deleting or expunging offensive posts. How frequently would you be willing to spend 50 minutes or so going through a thread, removing posted content, and PMing the poster regarding said removal? It takes time. You may be at work or at home on a weekend; want to spend most of an hour cleaning up a thread? If a thread gets that far gone, repeatedly, it is nuked and a PM is sent to the OP.
When a thread is disappeared, there is no placeholder that serves to say "This thread went wonky and offered no socially redeeming value whatever due to X". We would in effect have to start another thread to announce the demise of the previous thread, thereby offering a ready forum for the same sort of bozonity that was manifested in the thread which disappeared.
As in all things, decisions are made along a continuum that seeks to weigh how much value a thread offers to the board against the amount of supervision or oversight required to sustain that thread. If it's relatively self-sustaining, no issues.
If it's consistently going into the gutter, then someone needs to ask how much of their time and effort are they willing to spend to clean it frequently, send out PMs, and deal with those who can't stifle themselves from posting un-allowed content.
Your mileage may most definitely vary...
Doc