My number one reason would have to be fan bases. Now, this reason can be flexible. If the fandom ends up getting smaller over time/a lot of people leave it and the fandom becomes inactive, THEN I might consider joining. But usually what steers me away from series is the fandom because:
A. People are toxic to each other, because apparently not liking a ship warrants a death threat.
B. People are toxic to the creators, because apparently not making a ship canon warrants a death threat.
C. Too much drama -- which is A and B all rolled up into one PLUS some more. Rumors/Scandals about content creators, the actual shows' creators, blah, blah, blah. Like, if all I see if drama when I'm trying to search for the fandom, even when I use very specific tags to get AWAY from it, I just can't. I'm dropping the series and leaving it for another time, if I ever decide to come back.
My only exception to this rule will always be Tite's Kubo BLEACH because most of that drama is about the Bleach ending and the ships (IchiRuki versus IchiHime), and I can easily bypass all of that bullcrap with ease.
My smaller reason? -- Boobs and Gore. This is more specific to horror anime/manga series, but I will just drop a series if there's this weird combination of nudity and gore. If it's not someone getting murdered horribly, every. other. panel. is some shot of a girl's panties, cleavage, or some weird pose she's in for some reason showing BOTH. Lord help me if they show her exposed WHILE getting murdered. Hell, even a lot of the horror slasher movies don't amp it up like this.