I found it when the first fnaf released, I just loved how the game was scary and subtle, there was no need to gore or explicit stuff to be scary.

I don't say this because "oh with no gore is more friendly, good", I say this because even seeing more explicit stuff, I was amazed how it give me chills.

"A horror game that is not fill with gore and explicti stuff, was able to give me terror...I love it!"

Sure it's not the only one and there were other factors, like the concept, the gameplay, etc..

However that's the main reason