Time. Time multiplies costs. Time is the destroyer of worlds. Mismanagement and corruption can burn time, but in general I think much bigger factors are regulation, which requires coordination among distant, unmotivated entities, difficult even with good project management; and 2) inevitable bureaucratic and democratic squabbles that continually impose new hurdles and regularly move the goal posts.

Also consider inflation. If a project is 5 or 10 years late, simple inflation can blow up reported budgets by large fractions. When people report that California High Speed Rail will end up costing $100 billion, like 30-40% of that is simply counting inflation-adjusted costs at the time of expenditure for the extended timelines. But most people think that's $100 billion in today's dollars.

My rule as a voter is that once a project is approved, I'll vote against or oppose any modification to the project, even if I didn't want the project or would really like the modification. I realize there are often rational reasons for modifications, but in public works the risk of burning too much time is simply too high. No project will be perfect; just get it done, already. If it made sense in year 1 when approved, it should make as much sense in year 20, otherwise it should never have begun.

Probably all of them. We tend to think corruption first because the guy that was highest authority in the region for decades (and the capo of independentists mafia) is charged with accusations of a massive and systemic institutional corruption, that extracted 3% of every public contract.

The guy has been found to hold foreign accounts with astronomical amounts. That's not even controversial, he's more or less confessed, but he's not jailed yet after years of investigation and mountains of evidence.

Still that's not 3%, but multiple 100%'s.