In Buffy - The Vampire Slayer, vampires are demon spirits inhabiting a human body:
The canonical vampires, introduced in the first episode of the television series, are demonic spirits that inhabit human corpses.
From the Wikipedia Article on Vampires in the Buffyverse
In the Buffyverse, vampires cannot enter into a residence of a living being without being invited:
Vampires cannot enter a human residence without having been invited once by a living resident
Again from aforementioned Wikipedia Article.
As is seen in Episode 9 of Season 5, a demon can enter a house without being invitited. As it is stated in this question, in some mythologies, supernatural being cannot enter a house without being invited:
Some of them include the idea that not only vampires but supernatural beings in general can not enter the house without an invitation.
Has it ever been stated why a vampire cannot enter a residence without being invitited but a pureblood demon can, specially since a vampire is a demon spirit in a human body.
I am looking for an answer either from within the Buffyverse or a person being affiliated with the creation of it, e.g. Joss Whedon or a comic author explaining in an interview why it was decided to use that configuration.