Because that made some of the early vampire stories more interesting.
I've always found the "stake through the heart" method of killing a vampire quite interesting.
I'm prepared to bet that it also works on non-vampires!
Why can't vampires come in your house, unless you invite them in?
Its a metaphor for evil. Evil can't come into your life unless you invite it, according to the legend. Nowadays this vampiric legend has been replaced with tales of scary children with black eyes trying to get invited into people's homes.
They're just so darned polite.
i let skunks in my house why not vampires
Perhaps, it is merely linked to the concept of the Threshold Covenant . . Or Jewish Passover. The symbolism of a home's entry way that can be protected by the blood of a lamb so that the Angel of Death left those within alone.
Although, some of the plagues were not considered "evil" but mere "calamities", the Hebrew word Rah when used in certain context, does in fact mean "Evil" as in Joshua 45:7 - In this reasoning, the Angel of Death would be one of the evils let loose by God, and the threshold of the believer whom is either baptized or Born Again, would be symbolically protected by the Blood of the Sacrificed "Lamb of God" . . . Jesus Christ. So then, the option to "Invite them in" is Symbolic with Free Will letting Evil into one's house.
Many considered that a Vampire visiting the house to be a type of Plague or Curse, since this would often reveal itself in producing several sick family members (the vampire returns nightly), until usually the third night at which time that family member dies, being completely drained of blood. . . . Then Rises again as the undead . . . Another symbolic religious connection perhaps?
Mostly just speculation when applying Religious Myth symbolic meaning with other super natural myths representing evil.
Because some legends say evil cannot come across the threshold unless it invited.
They could curse your home.