Because there was a perception 50 years ago that Italians were dirtier than Northern Europeans. They may not be saying that now, but there is still stereotyping between parts of Europe, and the claims that this saying is impossible ring hollow to me.
I don’t have a smoking gun of someone saying it; but I do have a smoking gun of someone expressing the sentiment. That someone was Greek, and in fact, he was expressing annoyance at how clean Austria was compared to Italy.
Nikos Tsiforos. Gulliver in the land of the Giants (humorous travelogue through Central Europe). 1967. p. 12.
Why does everything have to be so clean? A Southerner will never understand this. Over in Tarvisio, ten Italian paces from here [Arnoldstein], there’s waste paper, filth, dust, leftovers from horse and cow hindquarters. Tourism pleads: “keep the area clean!”, but noone pays any attention—except for the pine trees, who are law-abiding citizens when they’re up on the mountaintops. Here in Arnoldstein, it’s as if they’ve made 300,000 Austrians lick the road clean. There’s not one piece of rubbish. Austria should be ashamed of how clean it is.
He says pretty much the same crossing the Swiss border into Italy at Valpelline.