WMG / Soul Train

Soul Train went into reruns because they ran out of new R&B/Soul they could dance to.

You can only play so much Beyoncé or hip-hop per show. Alternatively they couldn't book anymore guests after awhile. Hell they couldn't get people to show up for the award show. And them giving Ashanti a lifetime achievement award sealed it's fate.

It's cancellation was petty
The REAL reason it was cancelled was because people didn't want it surpassing American Bandstand.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/SoulTrain