Roman citizenship came with important civil rights.

Roman civilization since the Republic was legalistic and other tribes started to notice the advantages of it but trying to replicate it seems was not easy. So they wanted to the part of it.

Legalism could be rigid and excessive some times but it gives some predictability.

Italy was unified in Roman times as a single political unit. It was broken was germanic invasions, then Byzantine, etc.

Geography plays a part. The Apenines cross through all the peninsula.

Mountains even today creates difficult for transport, communication.