I think, but I might be wrong, that when you change families that action puts a stop to certain activities, wishes and opportunities. So I am assuming that parties is one of the things it stops.
"A hundred years is a mere blink in a life of an Elf. I'm patient. I can wait."
"I can see you know nothing of wizards. They are like winter thunder on a wild wind rolling in from a distance, breaking hard in alarm. But sometimes a storm is just a storm."
Also if you have a party on a lot that isn't your home and guests are there, then you go into Edit Town to add something to that place, when you return the guests will have all gone home with no recollection of ever having been invited, cause they aren't coming back.
I see it as the game protecting the little simmies by moving them to a safe place while you edit - or shift family.