Personal opinion, but even tho Demon Slayer is quite a simple story of good vs evil, the thing that makes it stand out is how the story and characters are handled;
Oh they use breathing? Everyone can breathe. Wait. HOW DID THEY DO THAT WITH ONLY BREATHING??
Oh yeah, another character's sad backstory -- oh that explains how they act and this gives their characters more depth.
The only thing that makes the series popular is the animation -- wait that's quite interesting, tell me more.
Ugh why is the MC like that so weak. Oh wait I am so used to the vengeful and stoic MC that I am baffled by a MC that strives with kindness because being kind is very hard in a harsh world.