Here's why:
* Pernell Roberts' Departure was Unexpected: Roberts' departure was not planned, and it shocked both the producers and the audience. The show was already immensely popular, and replacing a major character, especially one so beloved, was a risky move.
* The Show's Dynamics: Adam was a key character in Bonanza, and the show's dynamic relied heavily on the relationships between the Cartwright brothers. Recasting Adam would have fundamentally altered the show's core dynamic and likely alienated viewers.
* The Show's Focus: The show's focus was shifting to the younger Cartwright brothers, Hoss and Little Joe, as they matured. Recasting Adam would have likely shifted the focus back to the older generation, which was not in line with the show's evolving direction.
* The Show's Success: Bonanza continued to be successful after Roberts' departure, demonstrating that the show could thrive without Adam. The producers likely recognized that attempting to replace such a beloved character would be more detrimental than beneficial to the show's long-term success.
While there might have been internal discussions about replacing Adam, there's no concrete evidence that it was ever seriously considered. The show's success, the show's shifting focus, and the risks associated with recasting such a beloved character likely led the producers to choose a different path.