Has it ever been discussed why the "new" Supremes only made one appearance on the Ed Sullivan Show and Diana Ross [[solo) never appeared?

In the 60's, Diana and Mary, Flo/Cindy DOMINATED the show; after Diana left the Supremes in 1970, the Ed Sullivan Show was still on the air for another 12 - 18 months; in that time, The Supremes would have been on 5-6 times.

So what gives?