That is the result of her songwriting skills far exceeding her skills as a performing artist. She had one album that did well with the rest of her performing body of work being mediocre at best.She is in as a songwriter, not an artist. Her album Tapestry was the best selling album in history for quite a while. She should be in as an artist as well.
I think it will be a long wait before she is ever inducted as a performer. Album sales don't necessarily reflect quality, otherwise there would be Milli Vanilli, Spice Girls and much more trash among the inductees.