Why Did The Supremes Slip In 1968 ?

[Boogiedown]

For the year 1967 , The Supremes were #5 in Top Singles, #9 in Top Albums , and they were #6 and #4 in R&B Singles and R&B Albums.

For the year 1968, their annual rankings dropped to , #76 in Top Singles, #18 in Top Albums, and they were not visible at all in the R&B Top Fifty.

This data is from the book, THE WORLD OF SOUL, by Arnold Shaw which was written in the midst of the unfolding DRATS story and published in 1970.

What do you think would account for this shift ? In his book, Arnold Shaw provides three reasons he thinks are "worthy of consideration". Here are two of them:

1) Their hit cycle, short-lived with most artists, was over.

2) Hollland-Dozier-Holland, who were at odds for a time with the Motown management, had written themselves out or were unable to function effectively in the controversial atmosphere.

and reason 3) ..... well, before revealing what Shaw suggested back then , ... what would you say could've been reason #3?
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Note : The premise here is based on the claims of Arnold Shaw and the data he has provided , of which he doesn't cite the source.