Demographics are a major factor:

[S]outhern Europe’s economic disaster is both reflected — and is largely caused by — a demographic decline that, if not soon reversed, all but guarantees the continent’s continued slide. For decades, the wealthier countries of the northern countries — notably Germany — have offset very low fertility rates and declining domestic demand by attracting migrants from other countries, notably from eastern and southern Europe, and building highly productive export oriented economies.

In contrast, the so-called Club Med Countries– Greece, Italy, Portugal and Spain–have not developed strong economies to compensate for their fading demographics outside pockets of relative prosperity such as Milan.