Genoa’s Morandi bridge, which collapsed this week with the loss of at least 39 lives, was of exceptional strategic importance in Italy and beyond. The motorway it carried across the Polcevera stream was no ordinary road. It linked the two halves of Genoa. It was the corridor along which goods shuttled to and from Italy’s second-biggest port. And it was part of one of the main routes between Italy and France.

Why was the Genoa bypass not built? The answer tells us something about modern Italy

But why was this stretch of road, and the controversial 51-year-old bridge that supported it, made to bear such a heavy load – not just metaphorically, but physically, as the weight of vehicles increased over time?

Doubts about the design and construction of the Morandi bridge had been around for years. But so too had a plan to divert traffic away from it, taking the traffic to and from France in a sweeping 70km arc around Genoa. Why was it not built? The answer tells us something about modern Italy.

The simplest explanation is that it ran into vigorous opposition. As became embarrassingly clear this week, the opposition won enthusiastic backing from the Five Star Movement (M5S), the anti-establishment party founded by Beppe Grillo that is now the senior partner in Italy’s populist government.

In 2014 Grillo said the army should prevent construction of the Genoa bypass. And in a post to the M5S website – hastily removed after the disaster – the protesters had dismissed as a “fairytale” a claim that the bridge could give way. The resurrection of that post highlighted one of the biggest differences between M5S and its coalition partner, the far-right League: their irreconcilable infrastructure policies.

Much of M5S’s early support derived from groups of local activists, like those in Genoa, opposed to big infrastructure projects. Some were fighting schemes that represented a real threat to the environment. Others were inspired by pure nimbyism dressed up in green.

The League, by contrast, sees heavy spending on infrastructure as the way to revive Italy’s sluggish economy. It is an approach that also appeals to its electorate: traditionally composed of small business owners, but now increasingly embracing working-class voters who have suffered the direct effects of globalisation. Both groups warm to the rhetoric of the League’s bombastic leader, Matteo Salvini, and his Trump-like pledges of protectionism and the revival of old industries.

The government’s reaction to the disaster may well betray a desire to distract attention from this faultline in the coalition. Salvini first blamed Brussels for supposedly preventing Italy from spending on infrastructure, which is nonsense: it encourages it through the so-called Juncker plan. Then the government turned on the company that manages the country’s motorways, accusing it of failing to maintain the bridge (even though, according to a previous infrastructure minister, the state was jointly responsible). Most recently the M5S leader, Luigi di Maio, has picked a fight with the centre-left Democratic party over alleged contributions from the Benetton family, which controls the firm that owns the franchise.

Matteo Salvini, centre, Italy’s interior minister and leader of the League, visits Genoa after the bridge collapse. Photograph: Andrea Leoni/AFP/Getty Images

Another answer to the question of why the Genoa bypass was never built is rooted in the history of 20th-century Italy. After the fall of Mussolini’s fascist dictatorship, Italians created a democratic order in which no one person could wield absolute power. The result was a byzantine system of checks and balances that makes it almost impossible to get things done swiftly or decisively.

One of its key characteristics is the distribution, and overlap, of competencies among no less than four levels of government: national, regional, provincial and local. Projects of all kinds come to grief because while they may be backed by, say, the city council, they will then be opposed by the regional government – or vice-versa. Then there is the power of the courts to suspend work that has already begun so objections can be examined.

Italy is among the rich countries that spend most on road maintenance, and it is among those that spend least on road construction: since the creation of new infrastructure is so difficult, operators have little option but to patch up what already exists.

Yesterday it was announced that the Morandi bridge would be rebuilt. The operators of the motorway say it can be done in five months. Given the damage that will be done to the Genoese, and Italian, economies while the motorway remains closed, that is encouraging news.

But a new bridge will still mean that traffic that should be channelled around the city will be funnelled straight into it. The key question is whether the government in Rome will also allow work on the bypass to go ahead. And with an M5S minister holding the infrastructure portfolio in cabinet, that seems unlikely.

• John Hooper is the Economist’s Italy and Vatican correspondent