Why cant vampires come Into your house unless you welcome them in?

This is not a universal rule concerning vampires but was a mannerism made famous by Count Dracula when he deferred to Rhienfeld's invitation to have him enter the sanitarium. Just manners he retained from his education while still living.

Now the Count was not bound to this rule either. Certainly he entered Lucy Westenra's house when ever he pleased. When Dr. Van Helsing made effort to barr him from entering the Count employed a large wolf to do the door crashing for him.