Lorne Greene was originally signed to reprise his role of Ben Cartwright from the original TV series (Bonanza (1959)), but died before production began.
The reason the original cast doesn't appear in this film was that most of them were already dead. Only Michael Landon was still alive.
Hoss' death is finally explained as him drowning while saving a young girl. His tombstone reads 1842-1878.
The only remaining, living cast members at the time of shooting were Pernell Roberts (Adam Cartwright), Michael Landon (Little Joe Cartwright) and Bing Russell (Deputy Clem Foster). Roberts left the series after the sixth season for more challenging roles.
There was no mention of series' irregulars Sheriff Roy Coffee, 'Candy' Canaday, and Griff King in the production, nor their fates. Ben Cartwright and Hop Sing were mentioned, but not their fates.