By accepting Sam’s declaration that there would be no new King of Hell and by not challenging it, the demons acknowledged his authority. Sam may not be the king of Hell in name, but the hand-selected followers of the most recent contender to the throne literally would not dare to face him and fight for it.
“i have to think about the good, sam, because if i don’t, i will drown in the bad.”
— mary winchester, mirroring her son from over ten years later in 2.13 (there’s so much evil out there in the world, dean, i feel like i could drown in it)