If you base fan reactions and opinions as the reasons for its success/failure, then that conclusion is skewed. Basically what fans on the internet voice publicly is not what the majority of paying players feel. Fans on the internet (Twitter, Facebook, Instagram, reddit, wiki, etc) probably only account for less than 5-10% of Choices players who pay. What you believe is a treasure is another person's junk and vice versa.
1. While ES is popular in its own right, compared to books that came out around the same time frame (TRR and HSS), it actually did worse. ES book 1 had about 7.1 million views (from its debut to May 2020) while TRR book 1 had about 29.3 million views (from its debut to May 2020). While views don't translate equally to money made, it's reasonable that a book/series with more views (more readers) is more likely to have players spend on it (since they appear to be re-reading it). Compared to TRR, both ES and TRR only retained about 8% of their viewers from book 1 to book 3 and while that isn't good for the series, the sheer number of readers who started TRR vs ES helps TRR's numbers (TRR3 2.4 million vs ES3 600k); and is the reason TRR went on to start TRH.
2. While it may be hard to believe, while Open Heart 1 had the most views for 2019, BaBu book 1 had more monthly views than OH1. That is, OH1 was out longer that BaBu1 when the data was collected, so it had a head-start on the number of views. Again, this doesn't necessarily translate to money but more views (even if the majority of fandom uses the book to diamond-mine) usually translate to more players spending money. That is the reason it got a sequel. It's also a simpler easier-to-write book that reuses resources from other books. I think that also plays a factor on how profitable a book is.