Italy had 3 quirks when it came to ship design:
1.) A bunch of Italian cruisers ran a confusing line between destroyer and cruiser size, and were developed as counters to the overly-large French destroyers like the Le Fantasque and Mogador classes. As destroyers, their armor isn't quite so bad (just shy of 1 inch is still bad, but DDs aren't known for armor).
2.) Italy really wasn't planning on fighting a major war, and only joined WWII because the Allies looked like they were about to collapse in 1940. As a result, Italy ran into some serious problems when designs that gave them statistically-good ships--or more ships--ended up screwing them in wartime.
3.) Italy, as well as France (who was their main rival, and the nation they designed their ships to counter), really liked fast ships.
This at least isn't a historic flaw, the 1922 cruiser engine isn't based on interwar cruisers. Almost nothing produced in the interwar period did less than 30 knots, and the 1922 engine only really represents pre-WWI cruisers and coastal defense ships.