Wasn’t an adult at the time, but a bit too old to get into it either. So I didn’t have the fully fleshed out perspective, but the impression I got was that it was drawing in adult readers who weren’t already fantasy readers and ergo not familiar with the tropes and stereotypes of the genre. So the unoriginal elements appeared novel to them.

As far as why this particular series: my best guess is that it came down to the marketing. Maybe invoked some nostalgia in adult readers for older novels/series like Narnia.