Living in the area at the height of the protests, so I can fill in.
Some of the reasons: many people (including experts) argued that the project could have been done for half the price with almost the same effect by upgrading the existing station above ground instead of building an entirely new underground station, for example. Costs kept increasing – nothing new for big public infra projects, of course. But when a multi-billion euro project slowly triples its budget, people start asking questions.
That way it also took away funding from other smaller necessary projects. One should consider here that DB (railway operator) has been shutting down smaller, rural lines for decades making it harder and harder to rely on them, when you don't live on the main intercity network.
There were ecological concerns about the planned changes to Stuttgart's inner city layout and how it affects the already bad micro climate.
Plus there was a general sense of the project being pushed through by stubborn DB officials and state government as a kind of vanity project despite the aforementioned concerns. They acted completely tone-deaf to the protests and in one instance used excessive police force to crush a peaceful assembly. Just altogether bad topics, which did not make the project more popular.
I would add that the train station part freed up a lot of prime real estate in Stuttgart's city center by moving the railways, and the station, underground. I always had the impression that played a big role for everyone involved (DB, the city, politicians,...) in the decision to not budge on the train station part. The rest of Stuttgart 21, all the new tunnels and bridges and railway lines, are quite reasonable IMHO.
> Plus there was a general sense of the project being pushed through by stubborn DB officials and state government as a kind of vanity project despite the aforementioned concerns.
A public referendum was held in 2011. 58.9% voted for the project to be continued.
A state-wide referendum was held on the entire project, including the hundreds of kilometers of new tracks, tunnels, and bridges, a long-distance station for the Stuttgart airport, and a new station on the Swabian alb. There was never a referendum only on the new main station.
Germany has a history of poorly managed public projects. From the big projects in the last decade, there is not just Stuttgart 21, but also the new Airport in Berlin and the Elbphilharmonie-concert hall in Hamburg which are infamous for taking too long building-time, wasting too much money and offering a poor job for what they are supposed to do.
In similar vein there are many smaller projects which are just waste of money for no benefit. There is even a whole tv-series of such obscure failed projects, as also regularly reports of the fails of Germanys Government in handling Money. So Germans have very low trust in public projects, especially when they are expensive.
I guess such views do exist in most countries to some degree, but this is something that specially triggers German mentally, which usually more composed and on the peaceful side. Similar behavior surfaced in the last year with the different pandemia-rulings, and now with the vaccinations.
