If you went back to the 15th century and you told every one you came from the 20th century, would anyone beleive you? I guess that there is a lot of people who had travelled backwards, but we just don't know about it.
Well think about it really, If someone came to you and said that they were from the year 3000 would you believe them? Whose to say that really there were supposed to be large disasters occurring but didn't because when it happened they went back and stopped it from happening?