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 actors' discontent with the series' direction led to their departure after six seasons, which was subsequently revealed in Season 7 as being due to Cory and Trevor's institutionalization and subsequent discouragement from returning to the show.