Because it was only funny the first time

Yeah, when someone cracks a joke at a party and then someone says it again right after following a laugh, it doesn't always go so well.

Maybe the second poster just needed to be less complete... One of my high school buddies would do this (only once per audience, granted).

I would tell a joke. People would laugh. Then he would start telling the same joke, trailing off when people started looking at him funny, ending with "Well, it was funny when HE told it!"