I am one of her biggest fans, bitch is in my trinity (along with Jake Bugg), but sometimes I'm like wtf?!
I guess it is due to the fact that there are less reviews of her music
Acclaimed music accumulates reviews worldwide while Metacritic more specific in US, UK and Canada. Since her first 3 albums were country which unknown in many countries so they got less reviews than Madonna and others. That's why her ranked was lower until 1989 which pop and more acceptable worldwide and it got many reviews.