I believe one reason that is rarely talked about is that having children later in life makes you more tired than if you had them in your 20s. In the times when people were having more children, they were having them early. We all now how it is easier to shrug off a sleepless night in your 20s than in your 30s. We use that advantage to stay out drinking more when we are young, but it was probably evolved to help stay up at night when children were crying. So I think our cultural child bearing age is now misaligned from our biological adaptations, it feels exhausting to have children in your 30s
All italian dialects derive from a single language, with very low influence from the outside. Most Italian dialects are actually intelligible to an Italian speaker in written form and become difficult to understand only because of pronunciation. Try reading a few Wikipedia articles in different Italian dialects and you'll realize you understand at least 90% of text for all of them, possibly with the only exception of Sardinian.
Additionally, dialects were never the language of higher culture anywhere in Italy, all intellectuals have always used a common language and had a nationwide audience even when the country was politically fractured in many states. Said common language being Latin for many centuries after the fall of the Roman empire and Italian from around year 1500.
Sure it unified again only in the 19th century, but it has had a great degree of cultural union throughout its entire history. It is actually one of the most homogeneous countries in terms of language, religion and culture. Italians like to think otherwise for some reason.
Sorry for the digression!