Honestly, I think this is as much a question about Italian culture as it is about any Christian belief. The Italian culture produced laws against blaspheming saints; in the USA there were sin taxes and then the prohibition movement. I'm not sure that the reasons why are directly Christianity-related.
That said, one can point to common beliefs that influenced the practices. In this case Christians believe:
First, it falls generally under the umbrella of obscenities. Here is a previous answer regarding "bad words."
Secondly, to speak disrespectfully of those to whom one owes respect is wrong. It is, among other things, a violation of the principle of love. The ultimate transgression of this is to speak disrespectfully of God. Another example is to disrespect one's parents. Both of these cases make it into the Ten Commandments. These two cases are given special attention, but the general principle holds.
However, as to the laws themselves, I'd put them down to human nature in the context of a common moral code. Compare with modern attempts to outlaw "hate speech."