To many non-Italians, Matteo Renzi’s decision to make constitutional reform his political priority on becoming prime minister in 2014 was incomprehensible. He had taken office with substantial political capital to invest: He was a young and energetic leader who had shown remarkable political skill in seizing control of his party and the country and who was untainted by past scandals and compromises.

To outsiders, there appeared to be far more urgent challenges facing a country with the third-highest burden of public debt...