After Hannah Montana the Disney execs thought that every show should be music based so they got terrible actors who could kind of sing to star in these tv shows and it just didn't work. Then they had no good ideas and literally made the worst premises for shows. Like at least Nick shows were actually kind of funny. I remember laughing at iCarly and Victorious all the time.
It happened with the movies too after HSM. Camp Rock and Jump In are atrocious.