Most fans don't realize he was trying to sell other shows beyond Assignment: Earth. And Roddenberry was jonesing to get out from under Trek. It's part of why he handed over producing to Gene Coon in the first place. We've seen letters from him to his agent, etc., that make this rather plain, IIRC even before the 3rd season timeslot change to 10pm. @Harvey might remember better than I. Also, he had the opportunity to go over to National General and get a big paycheck for writing his ill-fated Tarzan script.