When Star Trek: The Original Series debuted on NBC in 1966, the series had already survived setbacks that killed other series. The series quickly became a sensation, but the budget was too high and the ratings weren't high enough. After battling creator Gene Roddenberry, NBC scheduled Star Trek in a graveyard timeslot for Season 3, and it was canceled. However, the show survived in syndication, and its legion of fans practically willed the franchise back into existence more than once.
Source：https://www.cbr.com/why-star-trek-was-originally-canceled/
