I think part of it has to do with how much ST was engrained into pop culture for about over half a decade beginning in 2018, especially riding off of ST2 as ST3 was aiming for 2019.
I also think part of it is that, scale wise, it's actually much more difficult to really capture ST in a way that makes the horror stand out. And that, unfortunately in a way; Hollywood's versions of the Stranger Things houses were really not good in a way that actively made HHN peeps on the west more hesitant and more weary on it.
And even with the advancements and time that came from ST2019 to ST4 in 2023, there still lies a weird feeling that Stranger Things...honestly shouldn't be at Halloween Horror Nights? If it was something that came around sooner--I'd argue Stranger Things would've probably of been more ideal for Fan Fest Nights as opposed to the latter (especially if FFN was bicoastal).