They're not
as funny and we all know that

.
Very true. The Japanese are, from what I have heard, a lot worse. I have never been to Japan, I honestly admit, though. But three people who have been there (and who are quite objective in these matters as one of them is a PhD and they usually don't make stuff up) tell me it could be a nasty experience for me as I've got extremely blue eyes.
Racism in Italy is a phenomenon that is misinterpreted by a lot of people. Italians for example will always mess with foreigners. Just yesterday I saw some waitress of a restaurant in an airport terminal make fun of people because they didn't speak Italian. The real problem obviously was that she didn't speak English and was working in an international airport.
And it doesn't matter if these foreigners are black or white. Nor does it matter if they are actually foreign. Sicilians will be ridiculed on a regular basis. Most Calabrian students in Bologna will never speak with a Calabrian accent because they are ashamed of it as well. Most of the time everyone is just joking, but it can go wrong very easily. I don't know what makes them think so, but if you're not from whatever village they were born in you don't seem to be as important as they are.
Apart from the campanilismo I see one other big reason. And that's the fact that immigrants from the South or East in Italy are poor. Africans making their way to Reggio di Calabria really aren't going to make their fortunes there. They end up selling rubbish on the beach and clamping on to people on the streets. If your only experience with immigrants is giving them money because they are dirt poor, you're going to become a wee bit racist no matter what. The Italian government obviously doesn't deal well with the issue though and it's clear for all to see that you can't really blame Africans and Eastern Europeans for trying to better their lives.
You could also go with Osman's theory.