Good one.  Thanks.

Here's the thing, she sold all those many, many MOR (Middle of the Road) albums during these "eras":
1. Folk Rock
2. R&B/Blues resurgence
3. Acid/Psychedelic Rock
4. Bubble Gum Music
5. Country Rock
6. Heavy Metal
7. Reggae
8. Rap/Hip Hop
9. Glam Rock
10. Arena Rock
11. Punk Rock

One of the secrets, I suspect: she didn't have much competition in her easy-going MOR niche. Many performed and recorded similar material, but made it bigger in another genre (Elton John would be one.) Regret that this sounds like "damning with faint praise," but many of us can name excellent "acts" or artists that never sold much of anything, simply because the competition was just too fierce in their own niche; the grind was too tough. I know of one, personally, who released 3 Canadian singles (one went to #10 nationwide), an album (which he helped write and produce himself) and gave it up.  He was a postman for over 30 years...