Bowles and Jackson left the show after the sixth season, due to their discontent about the direction of the show; their departure was explained by the announcement in Season 7 that Cory and Trevor had been committed to a mental institution and were advised to never return to Sunnyvale.
The performers' discontent with the series' trajectory prompted them to leave the program after its sixth season; this was subsequently revealed in Season 7, which stated that Cory and Trevor had been institutionalized and were advised against returning to Sunnyvale.