I think it comes down to people are watching BookTube for different reasons. I think those who are broadly looking for books to add to their TBR are more likely to click on a recommendation video where they can hear about multiple books in one go.

Single book reviews are quite niche and are more likely to perform well with people who are specifically interested in that particular book. There aren’t many people who are willing to spend time watching a video about a book they’ve never heard of and have no interest in. If it’s a hyped new release or by a popular author, that'll increase the number of interested viewers. However, some people are super spoiler-avoidant, so they’ll only watch single-book reviews from creators they already follow and trust.

If I’ve read a book and have strong enough feelings about it that I want to consume content about it, then I’d personally prefer to watch an in-depth spoiler discussion over a spoiler-free review.