Yeah, I think it's very much because it's a minority opinion. On those occasions when someone describes AtS as the better (or, more frequently, more mature/adult show), BtVS fans (myself included, perhaps sadly) tend to get more indignant/defensive and then make the comparison explicitly. It's just that while most discussions of BtVS are about BtVS, I feel like most of the popular press surrounding AtS is either about how it's a pale shadow of BtVS/it's better than BtVS/etc. I think it's something that is endemic to spinoffs, particularly less-popular spinoffs, and particularly particularly spinoffs that maintain closeish ties with the original show. There are much more people who have only watched BtVS and never bothered with AtS than the reverse.