Cancellation
In February 1968 while "Junkyard in Space" was in production, the cast & crew were told the series was picked up for a 4th season & new scripts were being written. Less than 3 months later, May 1968, CBS announced the TV series they were not renewing for the 1968-1969 season, which included "Lost in Space". Although CBS programming executives failed to offer any reasons why Lost in Space was cancelled, there are at least five suggested reasons offered by series executives, critics and fans, any one of which could be considered sufficient justification for cancellation given the state of the broadcast network television industry at the time.
Read more about this topic: Lost In Space