Bill Hobbs has a very good historical overview of the roots of merit selection in TN. In providing the overview, Hobbs asks a pointed question about the state’s judicial selection system:

If the current system, enacted by the legislature statutorily after voters rejected it as a constitutional amendment, is constitutional, then why was a constitutional amendment ever thought necessary in the first place? 

Hobbs also provides the answer: because the legislature didn’t believe Tennessee’s merit selection system was constitutional. A lawsuit is currently pending in federal court that challenges the Tennessee Plan as unconstitutional.