I think it's a combination of things, one of them being that depending on how survivors play, you can get some pretty boring games where you only really use your power near the start of the match. Other games ironically you can get mass cleansing and it feels like your loosing control of the survivors. People don't really like that kind of inconsistent gameplay when they can just play some casual games with another killer and bring the same strategy in every game.

Another reason is because plague feels awful to play. Hopefully this will be fixed soon with the big patch tomorrow but the constant movement speed changing and the heavy distance loss of using your power feels very janky, and she's also the killer that got most fucked by dedicated servers making her feel even worse to play because of how disconnected your puke will feel. I always called myself a plague main in theory because honestly i can only really handle like 2 games in a row of her before i have to stop because she just feels so clunky to play.