It's the same as to ask why some TV series get much more attention and wider audience compared to others, even if they are not necessarily that good. They just appease to the wider audience. Same with Booktok. Most books are easy to read which means they attract people who aren't generally so much into reading overall or that specific genre. But also people who are maybe avid readers but are looking for something light. And well..some will read them simply to see what is all the buzz about.

In some other cases, books become popular because most accounts (as far as I have seen) are dedicated to specific-genres or even qualities (smutty, cozy, etc). So it is likely that followers will enjoy the recommendations because they are in a way tailored to them, which in return means that they will be more proned to suggest them to others as well.

And well..some authors are great at creating social media content. I have seen several books becoming popular because author managed to create the hype. For example, Alex Aster and her Lightlark.