more than any other reason people name here, it's because of daily updates of several chapters, sometimes 3 chapters a day. This daily entertainment becomes the norm and is addicting to readers. Most readers that spend money are working adults with little time, so most of the time they can't do much other than checking their phone and are too tired to play games, so they either read webtoons or go to webnovel to read novels with thousands of chapters. It's all about infinite content, no matter how much you guys say that the site is shit, the prices are high or something, the content is literally endless and in much more robust quantities than Scribblehub (and where most of the time the novels aren't dropped as often), and obviously even more content than royal road too (where any good novel gets stubbed all the fucking time and they upload like a chapter a week lol).
I'm a Webnovel Author that's been there for 4 years by the way, with more than 10 novels being published (five of them are complete) and it does indeed bring lots of money, it's a rising market that will probably continue growing. However, because of this it is also becoming harder for new authors to find a spot and most of the time their novels flop and earn nothing. Webnovel readers standards have become much higher than before. I'm one of the lucky ones that started earlier and built a fanbase that will always read whatever I write to an extent (as long as it matches their prefered genres) but any new author is going to struggle much more unless they get a lucky golden egg and then become a one trick pony (can't make any other successful novel after their first one)