I think I read/heard that the Cowsill father/manager was very tough to work with.
Something like, they were on Ed Sullivan (could have been Smothers Bros or something else) and he was such a problem they decided to never have them back. Word got around and, eventually, nobody else would either.
If I’m wrong about this (I don’t think so) I will mea culpa with all due humility.
The Cowsill father was a horror. He also tried to rape his young daughter, Susan (and other young female members of the family).