KoraL Those books have chapters in the hundreds to thousands. Just paying for example, $30 a chapter for say 700 chapters is $21k, you do the math for those with even more chapters. And $30 is already at 10% of the standard professional market rate. If you want perfect, Three Body Problem quality, each book will be about six-figures. No publisher can afford these rates, nor would they see it as a worthwhile investment (very poor ROI, if any).
Trial Reads doesn't earn Webnovel much money, since the Coins are mostly refunded. It's also not meant to. Trial Read is an investment to find commercially viable books. Commercially viable means that people are willing to pay for them with actual cash. Statistics have shown that readers seldom even read past 3 chapters to decide whether to continue the book, so it doesn't matter if it's 40 or 60 chapters. Lowering the number of chapters lowers cost, speeds up the translation time, allowing them to offer more varieties to the paying readers. 1000 paying readers in Trial Reads is a super-hot book. Generally, you will see 100-200 supporters for books in Trial Reads. Anything above 400+ is considered a break-even. Generally, that number will rise dramatically once the book is out of Trial Reads; hence, the investment payoff.
If your favorite book isn't picked, it's simply because not enough people supported it with Coins.