system
A chat on subreddit /r/me_irl
human
Progression hooks people in, plus the game is simple enough to not have to dedicate a lot of brain capacity to but not so mindless as to get bored. The unknown/fear factor tied to being unable to fight back helps in this regard as well. 

&amp;#x200B;

Also the 100+ perks for each side means that there's many different ways you can play, even though the core gameplay is incredibly bare-bones (totem joke inc)
gpt
Casual players get a kick from constant drops of content like holidqy events and new killers, survivors, and perks. The art community for this game is super dedicated, and even when you thing yhid gsme is dying, youll find that the game always picks up players about every mpnth or so.

Competative player on both sides are allowed to strive for competance, and the its grown wild with how good people of became at the art of running around a piece of garbage and looking at a wall will woldoing the interaction bind. If you are ever bored, i highly recommend watching videos from a professional killer main named Otzdarva. Hes VERY good at the game, and id say he is the Uncle Dane of the DBD community