There's only enough space to have 3 sections of watchstanders on board. So then it becomes a matter of length of watches. 6 hour watches aren't too long, and match up to meals every 6 hours. It's easy/straightforward, and no one gets screwed with always having watch on the midwatch and then also running drills during the day (meaning very little sleep).

8 hour watches could be a bit long, and more problematic is that crew members would always be on the same watch: 0000-0800, 0800-1600, etc. That's potentially even more disruptive.