Roddenberry originally pitched a half-hour
Assignment: Earth series proposal in late 1966, independently of
Star Trek. When the networks rejected it, he reworked it into a backdoor-pilot episode of ST to give it a second chance (hoping, I suppose, that a "demo reel" of the show would convince the executives where the proposal alone failed, and also hoping for a show to fall back on if ST didn't get renewed for a third season), but it still didn't sell.
Why not? Well, most proposed series don't sell. They're competing for a limited number of slots, and so only the strongest survive. The book
Inside Star Trek by Desilu executive Herb Solow and TOS associate producer Bob Justman says that as a pilot, "Assignment: Earth" was "well below average" and was "immediately discounted as a potential series by the network" (p. 373).
And I suppose I can understand that. Sure, it was fun watching Gary and Roberta's interplay, Gary's gadgets, and so forth, but the episode didn't really give a great sense of what Gary and Roberta would be
doing on a weekly basis. Sabotaging a US nuclear platform and almost triggering a war probably didn't seem like a heroic enough sort of thing to the network execs, and "to prevent Earth's civilization from destroying itself before it can mature into a peaceful society" was a little too abstract. At least in the original pilot proposal, there were clear antagonists to be fought.
Which is not to say that I haven't enjoyed the Gary Seven followups in the literature or that I'm not looking forward to John Byrne's take on the series. But I'm not a 1960s network executive trying to decide which shows get the limited amount of money my bosses are willing to let me spend.