My "guess":
Sometimes they are no longer needed because they are covered with OS updates or patches, or just aren't found in the wild any more?
Ping of Death is an example - nothing out in the mainstream is actuallly vulnerable any more so they could remove that (but have not!)
We keep getting FALSE ALERTS for PoD, and yet there's no OS currently in use that is even vulnerable for the last several years. So technically, even if someone TRIED a PoD against us, it would be futile.