CBS wasn't the sole reason UFO was canned. While CBS had it on late at night (that's where I saw it as a kid) the show was aired on Saturday mornings on London Weekend Television and apparently not aired consistently.
From what I'd heard, it had a bigger following in the UK per capita than it did in the USA. Back then, the aim was to appeal to the US market because of the enormous audience potential. CBS was a critical part of the equation, as it was the only network broadcasting the show. Most kids I met who liked sci-fi in 1971 didn't even know that UFO existed let alone was canceled. Thankfully it went into syndication shortly thereafter and that changed. I remember a few sci-fi get-togethers where UFO and Star Trek were the popular shows, despite that they weren't airing new episodes. It was just my personal impression that UFO could have done better had more people been aware of it. CBS screwed up on the demographic and aired it in too late a time slot, in addition to cutting corners on marketing.
It was on Saturdays at 7pm in the bay area. So the time was right, if not the day (then again, if they were aiming it at 8 year-olds, they'd've been in for the night.) I don't remember network programming as starting at 7pm every night either ... 7:30, after the local news, that I remember. Maybe Disney at 7 was the exception on Sunday? That would suggest it was in syndication ...
Well, maybe I'm full of it. I know UFO was on in 71 ... but I'm wondering if the eps I saw were in 72 ... did it go into syndication immediately? It was on the CBS affiliate, KPIX, I do recall that. My stepdad was crazy about it, I bough him that UFO INTERCEPTOR in metal that had a missile you launched with a cap for the bang effect. He had the thing still in the hard bubble wrap and box up till the early 90s.
Back on topic: The quote I remember reading about Harris turning down the LiS film role was 'I don't do cameos.'