Her books are not that good; she's much closer to Dan Brown in terms of her success than people would think. The right product at the right time, quality is not paramount. Same as the Hunger Games, or Twilight, or Goosebumps.
If it were quality, Diane Duane or many, many other skilled writers would see success.
If it were quality, Diane Duane or many, many other skilled writers would see success.