The three biggest ways that I've heard (https://www.hometheaterforum.com/community/threads/can-someone-tell-me-why-chico-and-the-man-is-mia.378739/) are either a really poor final season (https://www.reddit.com/r/television/comments/ga1btw/what_finale_or_final_season_ruined_an_entire_show/), if the show had two or three of its seasons in black and white (https://www.hometheaterforum.com/community/threads/episodes-of-classic-tv-shows-out-of-syndication.338339/) before switching to color, and if something really (https://www.sitcomsonline.com/boards/showpost.php?p=5126853&postcount=10) tragic (https://www.reddit.com/r/NoStupidQuestions/comments/n5oxs5/what_movies_or_tv_shows_were_ruined_by_an_actors/) or controversial (https://tvline.com/lists/tv-shows-biggest-scandals-goldbergs-empire-buffy/) happened to the stars in real life. It also probably goes without saying (https://movies.stackexchange.com/questions/3147/when-why-did-american-television-change-from-primarily-episodic-to-primarily-ser) that if a show (https://www.trekbbs.com/threads/episodic-vs-serialized.305638/) is heavily serialized (https://www.reddit.com/r/TheShield/comments/r44rgm/syndication_question/) or is too topical (https://www.reddit.com/r/television/comments/7ce15t/comment/dppe0dr/) then it probably isn't very desirable (http://kenlevine.blogspot.com/2015/02/shows-that-work-in-syndication-and.html) for syndication on its own.