The answer is in the letter left by Hojo in Tifa's room on her desk,if you read if of course then you should know that the people living there are from Shinra and are pretending that it never happened.Most likely so anyone cannot blame Shinra for everything that is happening and all.And until Cloud and his party get there it also says in the letter that no one from back then has come back so everything's ok.So even if anyone did come back in Nibelheim from that encident(like Cloud and his party) wouldn't really cause much of a problem since probably no body would believe them.