You know why? Because for some stupid reason the current re-runs on TV are made up of only the seasons that have been released on DVD (seasons 1 -4). I think the Anthrax episode was in like season 6 or something. I've looked for it myself.
The really weird thing is that past re-runs, from before the first time they were released on DVD, had all the seasons. I'll bet it's some kind of marketing thing.