I've wondered that, too. I guess it's because bods generaly stop at the Alps?
I think that is the answer: generally, if you've taken a scenic/minimal motorway route and ridden for two, three or even four days to get to the Italian border - and know that you have an equally long journey to get home - it puts a lot of pressure on the holiday. And when getting there means riding through some spectacular areas, whether it's the French, Swiss or Austrian mountains, it's hard to see the benefit in pushing on a lot further. Particularly if the bit of Italy in which you've landed is the Dolomites. A few days round there and then heading for home feels like time very well spent (because it is, of course). Especially as the contrast when you roll out of the Alps and into the Po valley is stark: congested towns and cities, flat and busy roads – turning round and heading back into the Alps suddenly makes a heap of sense!
The Italian races do help lure some people further south – into the mountains of Tuscany to visit Mugello, or over to the coast for San Marino (though to be honest you need to head away from Rimini for the good riding - but not very far). When the trains ran down to Livorno, that was a great opportunity to get a head start and explore more of the rest of the country - especially Tuscany, Lazio, Le Marche and Umbria.
But it is definitely worth exploring further south. Road conditions do vary - generally from immaculate to awful, with little that you'd call average in between - but the scenery is spectacular. You have the Apennines running down the spine of the peninsula spawning fabulous roads everywhere. Some of the riding in Calabria and Basilicata is astonishing.
For me, though, Sicily is the one bit of Italy I'm not bothered about visiting again. Bits of it are spectacularly beautiful, but bits of it are jaw-droppingly awful, from the litter in every lay-by to the beggars in every town and the constant suspicion that the locals are about to rob you or nick the bike. Also, it's the place I've felt least safe around local drivers – even the insanity of Naples didn't worry me as much. I've never seen a head-on car crash in the UK; on my first trip to Sicily, I saw two within about 40 mins of each other - cars nose-to-nose in the middle of the road with people (at one of them including a police man) standing around gesticulating. It is a shame because the north coast, Etna, down towards Syracuse.... well, pretty much all of the island is beautiful and the weather is normally lovely. Clearly, the secret to enjoying is going to be sticking to the nice bits while avoiding the rough bits (Catania!) but I've been there four times now and that's enough for me.
Sardinia, on the other hand... book me up now!